Add WithRPCClientDialer option to inject the execution node RPC client

[satushh]

What type of PR is this?

Feature

What does this PR do? Why is it needed?

• Today the execution service can only reach the execution client by dialing an address (HTTP URL or IPC path).
• When the execution client runs inside the same process as Prysm (embedded / single-binary setups), there is no address to dial. The host has to create a unix socket purely so Prysm can dial it: an OS round-trip between two components that share memory.
• This PR adds one option, execution.WithRPCClientDialer, that lets the host hand the service a function returning a ready *rpc.Client (e.g. from gethRPC.DialInProc) instead of an endpoint string.
• It is a dialer (function), not a client object, on purpose: the service's reconnect loops close the old client and dial a fresh one. With a dialer, those loops keep working unchanged: they just call the function again.
• If both a dialer and an endpoint are configured, the dialer wins. The CLI always fills in a default --execution-endpoint, so rejecting the combination would make the option unusable through the normal node startup path.
• Everything after the dial is untouched: chain ID verification, deposit contract wiring, and all Engine API code work on the injected client exactly as before.
• No behavior change for existing users: without the option, the service dials the configured endpoint exactly as it does today.
• No node package changes: the option reaches the service through the existing node.WithExecutionChainOptions.
• Usage from an embedding host: a single binary running geth and Prysm in one process enables this entirely at construction time, with no flags and no other Prysm changes:
  node.New(ctx, cancel, node.WithExecutionChainOptions([]execution.Option{execution.WithRPCClientDialer(func(ctx context.Context) (*gethRPC.Client, error) { return gethRPC.DialInProc(gethInProcHandler), nil })}), ...)
  (after which every Engine API call travels in-memory instead of over a socket).

Which issue(s) does this PR fix?

Fixes #

Other notes for review

Acknowledgements

• I have read CONTRIBUTING.md.
• I have included a uniquely named changelog fragment file.
• I have added a description with sufficient context for reviewers to understand this PR.
• I have tested that my changes work as expected and I added a testing plan to the PR description (if applicable).