[enhancement] add proto for opt args

[proppy]

What's hard to do? (limit 100 words)

Currently we can specify args as proto file for the following step in the toolchain:

• ir convertion: https://github.com/google/xls/blob/main/xls/dslx/ir_convert/ir_converter_options_flags.proto
• scheduling: https://github.com/google/xls/blob/main/xls/tools/scheduling_options_flags.proto
• codegen: https://github.com/google/xls/blob/main/xls/tools/codegen_flags.proto

But we don't have any equivalent to specify opt flags:

xls/xls/tools/opt_main.cc

Lines 71 to 151 in f571bc4

	ABSL_FLAG(std::string, output_path, "-",
	"Output path for the optimized IR file; '-' denotes stdout.");
	ABSL_FLAG(std::optional<std::string>, alsologto, std::nullopt,
	"Path to write logs to, in addition to stderr.");
	// LINT.IfChange
	ABSL_FLAG(std::string, top, "", "Top entity to optimize.");
	ABSL_FLAG(std::string, ir_dump_path, "",
	"Dump all intermediate IR files to the given directory");
	ABSL_FLAG(std::vector<std::string>, skip_passes, {},
	"If specified, passes in this comma-separated list of (short) "
	"pass names are skipped.");
	ABSL_FLAG(std::optional<int64_t>, convert_array_index_to_select, std::nullopt,
	"If specified, convert array indexes with fewer than or "
	"equal to the given number of possible indices (by range analysis) "
	"into chains of selects. Otherwise, this optimization is skipped, "
	"since it can sometimes reduce output quality.");
	ABSL_FLAG(
	std::optional<int64_t>, split_next_value_selects, 4,
	"If positive, split `next_value`s that assign `sel`s to state params if "
	"they have fewer than the given number of cases. This optimization is "
	"skipped for selects with more cases, since it can sometimes reduce output "
	"quality by replacing MUX trees with separate equality checks.");
	ABSL_FLAG(int64_t, opt_level, xls::kMaxOptLevel, []() -> const std::string& {
	static const std::string kDescription = absl::StrFormat(
	"Optimization level. Ranges from 1 to %d.", xls::kMaxOptLevel);
	return kDescription;
	}());
	ABSL_FLAG(std::string, ram_rewrites_pb, "",
	"Path to protobuf describing ram rewrites.");
	ABSL_FLAG(bool, use_context_narrowing_analysis, false,
	"Use context sensitive narrowing analysis. This is somewhat slower "
	"but might produce better results in some circumstances by using "
	"usage context to narrow values more aggressively.");
	ABSL_FLAG(
	bool, optimize_for_best_case_throughput, false,
	"Optimize for best case throughput, even at the cost of area. This will "
	"aggressively optimize to create opportunities for improved throughput, "
	"but at the cost of constraining the schedule and thus increasing area.");
	// LINT.ThenChange(//xls/build_rules/xls_ir_rules.bzl)
	ABSL_FLAG(bool, enable_resource_sharing, false,
	"Enable the resource sharing optimization to save area.");
	ABSL_FLAG(bool, force_resource_sharing, false,
	"Force the resource sharing pass to apply the transformation where "
	"it is legal to do so, overriding therefore the profitability "
	"heuristic of such pass. This option is only used when the resource "
	"sharing pass is enabled.");
	ABSL_FLAG(std::string, area_model, "asap7",
	"Area model to use for optimizations.");
	ABSL_FLAG(
	std::optional<std::string>, passes, std::nullopt,
	"Explicit list of passes to run in a specific order. Passes are named "
	"by 'short_name' and if they have non-opt-level arguments these are "
	"placed in (). Fixed point sets of passes can be put within []. Pass "
	"names are separated based on spaces. For example a simple pipeline "
	"might be \"dfe dce [ ident_remove const_fold dce canon dce arith dce "
	"comparison_simp ] loop_unroll map_inline\". This should not be used "
	"with --skip_passes. If this is given the standard optimization "
	"pipeline is ignored entirely, care should be taken to ensure the "
	"given pipeline will run in reasonable amount of time. See the map in "
	"passes/optimization_pass_pipeline.cc for pass mappings. Available "
	"passes shown by running with --list_passes");
	ABSL_FLAG(std::optional<std::string>, passes_proto, std::nullopt,
	"A file containing binary PipelinePassList proto defining a pipeline "
	"of passes to run");
	ABSL_FLAG(std::optional<std::string>, passes_textproto, std::nullopt,
	"A file containing textproto PipelinePassList proto defining a "
	"pipeline of passes to run");
	ABSL_FLAG(std::optional<int64_t>, passes_bisect_limit, std::nullopt,
	"Number of passes to allow to execute. This can be used as compiler "
	"fuel to ensure the compiler finishes at a particular point.");
	ABSL_FLAG(bool, passes_bisect_limit_is_error, false,
	"If set then reaching passes bisect limit is considered an error.");
	ABSL_FLAG(bool, list_passes, false,
	"If passed list the names of all passes and exit.");
	ABSL_FLAG(
	std::optional<std::string>, pass_metrics_path, std::nullopt,
	"Output path for the pass pipeline metrics as a PassPipelineMetricsProto.");
	ABSL_FLAG(bool, debug_optimizations, false,
	"If passed, run additional strict correctness-checking passes; this "
	"slows down the optimization significantly, and is mostly intended "
	"for internal XLS debugging.");

Current best alternative workaround (limit 100 words)

None

Your view of the "best case XLS enhancement" (limit 100 words)

• add a new opt_options_flags.proto definition
• add --opt_options_proto and --opt_options_used_proto to opt_main