Adds author_rotateKeys RPC (#3337)

* Adds `author_rotateKeys` RPC

`author_rotateKeys` will rotate all session keys, store them into the
keystore and returns their encoded public keys as result. These encoded
public keys can be used directly to send the transaction to the chain.

* Review comments

substrate/core/test-runtime/src/lib.rs

@@ -37,15 +37,15 @@ use substrate_client::{
 	impl_runtime_apis,
 };
 use sr_primitives::{
-	ApplyResult, create_runtime_str, Perbill,
+	ApplyResult, create_runtime_str, Perbill, impl_opaque_keys,
 	transaction_validity::{TransactionValidity, ValidTransaction},
 	traits::{
 		BlindCheckable, BlakeTwo256, Block as BlockT, Extrinsic as ExtrinsicT,
-		GetNodeBlockType, GetRuntimeBlockType, Verify, IdentityLookup
+		GetNodeBlockType, GetRuntimeBlockType, Verify, IdentityLookup,
 	},
 };
 use runtime_version::RuntimeVersion;
-pub use primitives::hash::H256;
+pub use primitives::{hash::H256, crypto::key_types};
 #[cfg(any(feature = "std", test))]
 use runtime_version::NativeVersion;
 use runtime_support::{impl_outer_origin, parameter_types};
@@ -434,6 +434,15 @@ fn code_using_trie() -> u64 {
 	} else { 103 }
 }
 
+impl_opaque_keys! {
+	pub struct SessionKeys {
+		#[id(key_types::ED25519)]
+		pub ed25519: ed25519::AppPublic,
+		#[id(key_types::SR25519)]
+		pub sr25519: sr25519::AppPublic,
+	}
+}
+
 #[cfg(not(feature = "std"))]
 /// Mutable static variables should be always observed to have
 /// the initialized value at the start of a runtime call.
@@ -612,6 +621,12 @@ cfg_if! {
 					runtime_io::submit_transaction(&ex).unwrap();
 				}
 			}
+
+			impl session::SessionKeys<Block> for Runtime {
+				fn generate_session_keys(_: Option<Vec<u8>>) -> Vec<u8> {
+					SessionKeys::generate(None)
+				}
+			}
 		}
 	} else {
 		impl_runtime_apis! {
@@ -816,6 +831,12 @@ cfg_if! {
 					runtime_io::submit_transaction(&ex).unwrap()
 				}
 			}
+
+			impl session::SessionKeys<Block> for Runtime {
+				fn generate_session_keys(_: Option<Vec<u8>>) -> Vec<u8> {
+					SessionKeys::generate(None)
+				}
+			}
 		}
 	}
 }