rpc server: add rate limiting middleware (#3301)

Add RPC server rate limiting which can be utilized by the CLI
`--rpc-rate-limit <calls/per minute>`

Resolves first part of
https://github.com/paritytech/polkadot-sdk/issues/3028

//cc @PierreBesson @kogeler  you might be interested in this one

---------

Co-authored-by: James Wilson <james@jsdw.me>
Co-authored-by: Xiliang Chen <xlchen1291@gmail.com>

substrate/client/rpc-servers/src/middleware/mod.rs

@@ -18,6 +18,10 @@
 
 //! JSON-RPC specific middleware.
 
+/// Grafana metrics middleware.
 pub mod metrics;
+/// Rate limit middleware.
+pub mod rate_limit;
 
 pub use metrics::*;
+pub use rate_limit::*;