Adds babe rpc support (#4729)

* babe_epochAuthorship
remove test-helpers from sp-keyring, bump spec_version, impl_version

* bump Cargo.lock

* add BabeRPC to node-rpc

* rename to BabeApi, remove err_derive

* pass &ServiceBuilder to with_rpc_extensions callback

* sc-consensus-babe-rpc

* Update client/consensus/babe/src/lib.rs

Co-Authored-By: Tomasz Drwięga <tomusdrw@users.noreply.github.com>

* Better docs, code style chanegs

Co-Authored-By: André Silva <andre.beat@gmail.com>

* new line at the end of Cargo.toml

Co-authored-by: Tomasz Drwięga <tomusdrw@users.noreply.github.com>
Co-authored-by: André Silva <andre.beat@gmail.com>

substrate/bin/node/runtime/src/lib.rs

@@ -83,7 +83,7 @@ pub const VERSION: RuntimeVersion = RuntimeVersion {
 	// implementation changes and behavior does not, then leave spec_version as
 	// is and increment impl_version.
 	spec_version: 220,
-	impl_version: 0,
+	impl_version: 1,
 	apis: RUNTIME_API_VERSIONS,
 };
 
@@ -737,6 +737,10 @@ impl_runtime_apis! {
 				secondary_slots: true,
 			}
 		}
+
+		fn current_epoch_start() -> sp_consensus_babe::SlotNumber {
+			Babe::current_epoch_start()
+		}
 	}
 
 	impl sp_authority_discovery::AuthorityDiscoveryApi<Block> for Runtime {