feat: add support for testing_ rpc namespace and testing_buildBlockV1

[0xKarl98]

This aims to reoslve this issue #20069 , summerized as below :

• Implemented the testing_buildBlockV1 RPC namespace (testing-only, default disabled) per spec:

  • Added TestingApi and TestingBuildBlockRequest (camelCase, payloadAttributes+transactions+extraData) in rpc-ap

  • Implemented server-side handler and builder: TestingBlockBuilder trait and EthTestingBlockBuilder execute only the provided raw signed transactions (order preserved), reject blob tx without sidecars, apply payloadAttributes/extraData, and return ExecutionPayloadEnvelopeV4 without touching the canonical chain.

  • Wired conditional registration: hidden CLI flags --http.testing / --ws.testing add testing to module selection, and in Ethereum node we construct EthTestingBlockBuilder and merge TestingApi only when the testing module is explicitly requested. Security note added to API docs

  • Expose a hidden --testing.max-concurrent flag (default 1), and wire testing RPC instantiation to use a semaphore + spawn_blocking so heavy block builds don’t block the async runtime and have bounded concurrency

References :
testing_buildBlockV1.md

cc @mattsse

[0xKarl98]

@mattsse Should we support testing blob/sidecar transactions here ?

[mattsse]

this looks pretty good already

left some suggestions

[mattsse]

these args we can exclude, we can make it so that --http.api also accepts testing

[0xKarl98]

Yeah we can exclude that and make it more simple

[mattsse]

let's name this V1

Suggested change

	pub struct TestingBuildBlockRequest {
	pub struct TestingBuildBlockRequestV1 {

[mattsse]

we can exclude this from here, and instead instantiate and install here

reth/crates/ethereum/node/src/node.rs

Lines 311 to 314 in a62b46a

	container
	.modules
	.merge_if_module_configured(RethRpcModule::Eth, eth_config.into_rpc())?;

[mattsse]

these changes we can all undo because we can instantiate this on demand specifically for eth

similar to flashbots api:

reth/crates/ethereum/node/src/node.rs

Lines 307 to 310 in a62b46a

	container.modules.merge_if_module_configured(
	RethRpcModule::Flashbots,
	validation_api.into_rpc(),
	)?;

[mattsse]

this entire fn logic we need to spawn as blocking, ideally also add a semaphor for this so we can limit how many requests we allow at the same time

[0xKarl98]

In terms of the semaphor , which number do you think is appropriate by default ?

I just set the concurrent number to at least 1 and can be overriten if value provided