Move beacon pool http api to its own separate module (#8543)

Continuation of:
* #8536

Moving `/beacon/pool` endpoints out of `http_api` to a separation module. This should improve code maintainability, incremental compilation time and rust analyzer performance.

This is a tedious but straight forward change, so we're going with a pair & insta-merge approach to avoid painful & slow async review


  


Co-Authored-By: Jimmy Chen <[email protected]>

Author: jimmygchen

beacon_node/http_api/src/beacon/mod.rs

@@ -1 +1,2 @@
+pub mod pool;
 pub mod states;