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

substrate/client/service/test/src/lib.rs

@@ -184,6 +184,7 @@ fn node_config<G: RuntimeGenesis + 'static, E: ChainSpecExtension + Clone + 'sta
 		chain_spec: Box::new((*spec).clone()),
 		wasm_method: sc_service::config::WasmExecutionMethod::Interpreted,
 		execution_strategies: Default::default(),
+		unsafe_rpc_expose: false,
 		rpc_http: None,
 		rpc_ws: None,
 		rpc_ws_max_connections: None,