RPC: Limit request payload size to 50kB (#12287)

(cherry picked from commit 32dcce0ac1)

Co-authored-by: Trent Nelson <trent@solana.com>

core/src/rpc.rs

@@ -79,6 +79,8 @@ use std::{
 };
 use tokio::runtime;
 
+pub const MAX_REQUEST_PAYLOAD_SIZE: usize = 50 * (1 << 10); // 50kB
+
 fn new_response<T>(bank: &Bank, value: T) -> RpcResponse<T> {
     let context = RpcResponseContext { slot: bank.slot() };
     Response { context, value }