RPC: add caching to getLargestAccounts (#15154)

* introduce get largest accounts cache

* remove cache size and change hash key

* remove eq and hash derivation from commitment config

* add slot to the cache

client/src/lib.rs

@@ -8,6 +8,7 @@ pub mod mock_sender;
 pub mod nonce_utils;
 pub mod perf_utils;
 pub mod pubsub_client;
+pub mod rpc_cache;
 pub mod rpc_client;
 pub mod rpc_config;
 pub mod rpc_custom_error;