Allow to expose a subset of unsafe RPCs (#5233)

* sc-cli: Use type-safe constructors for RPC/Prometheus interfaces

* service: Simplify rpc handler creation

Could probably be further simplifies once [this][commit] lands.

[commit]: https://github.com/paritytech/jsonrpc/commit/20485387ed06a48f1a70bf4d609a7cde6cf0accf

* service: Streamline some HTTP & WS server start logic

* client: Introduce a simple RPC policy mechanism

* rpc/system: Check unsafe RPCs

* rpc/offchain: Check unsafe RPCs

* rpc/author: Check unsafe RPCs