rpc: Add getextrapoolinfo RPC

[1440000bytes]

Rationale:

• It will help users to know about extra pool transactions
• Developers can use it while testing compact block relay

[kwsantiago]

The implementation is functional but has performance and documentation issues. The most critical gap is the lack of test coverage. The counting function's O(n) complexity could impact performance when the extra pool is large (default 32,768 txs).

[kwsantiago]

Holding cs_main lock while calling the potentially slow count function could block other threads unnecessarily.

[1440000bytes]

Thanks for the review! I’ve addressed some of the comments and will work on improving the code further.