Add support for filtering OpenAPI document

[simonjbeaumont]

Motivation

When generating client code, Swift OpenAPI Generator generates code for the entire OpenAPI document, even if the user only makes use of a subset of its types and operations.

Generating code that is unused constitutes overhead for the adopter:

• The overhead of generating code for unused types and operations
• The overhead of compiling the generated code
• The overhead of unused code in the users codebase (AOT generation)

This is particularly noticeable when working with a small subset of a large API, which can result in O(100k) lines of unused code and long generation and compile times.

For a more detailed motivation and design, see the proposal in #303.

Modifications

• Add document filter to the generator config.
• Run filter as a post-transition hook in the generator pipeline after parsing the document.
• Provide a CLI command that outputs the filtered document to stdout.

Result

Users can filter a document before code-generation. For large APIs, this can result in >90% speedup (see proposal).

Test Plan

• Unit tests.

[czechboy0]

Looks great overall, just a few small suggestions.

[czechboy0]

A few more comments, otherwise looks great.

[czechboy0]