make `OpAddOnsBuilder` generic over middleware

[thaodt]

Describe the feature

It looks like I still got the type constraint when injecting middleware, for e.g.:

110 |             addons.with_rpc_middleware(
    |                    ------------------- arguments to this method are incorrect
111 |                 SlowRequestLogMiddleware::new(self.slow_request_log_config.clone())
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `Identity`, found `SlowRequestLogMiddleware`
    |
help: the return type of this call is `SlowRequestLogMiddleware` due to the type of the argument passed

reth/crates/optimism/node/src/node.rs

Lines 285 to 291 in 36d568a

	impl<N, NetworkT> Default
	for OpAddOns<
	N,
	OpEthApiBuilder<NetworkT>,
	OpEngineValidatorBuilder,
	OpEngineApiBuilder<OpEngineValidatorBuilder>,
	>

I think we need to add Identity here and update the builder() method to be generic over RpcMiddleware

reth/crates/optimism/node/src/node.rs

Lines 301 to 308 in 36d568a

	impl<N, NetworkT>
	OpAddOns<
	N,
	OpEthApiBuilder<NetworkT>,
	OpEngineValidatorBuilder,
	OpEngineApiBuilder<OpEngineValidatorBuilder>,
	>
	where

Also, update the OpAddOnsBuilder to support custom middleware.

I have a working solution already, I'd like to take this if you're open to it! @mattsse 🙏

Additional context

relating to #17076

[ritik4ever]

Can take this issue
ETA 2 day