Expose information about the extrinsic in the metadata (#4774)

* Expose information about the extrinsic in the metadata

This pr exposes some information about the extrinsic used in the runtime
via metadata. The following information are exposed:

- Version of the extrinsic
- List of all signed extensions used by the extrinsic.

* Increment `spec_version`

substrate/primitives/runtime/src/generic/unchecked_extrinsic.rs

@@ -20,7 +20,10 @@ use sp_std::{fmt, prelude::*};
 use sp_io::hashing::blake2_256;
 use codec::{Decode, Encode, EncodeLike, Input, Error};
 use crate::{
-	traits::{self, Member, MaybeDisplay, SignedExtension, Checkable, Extrinsic, IdentifyAccount},
+	traits::{
+		self, Member, MaybeDisplay, SignedExtension, Checkable, Extrinsic, ExtrinsicMetadata,
+		IdentifyAccount,
+	},
 	generic::CheckedExtrinsic, transaction_validity::{TransactionValidityError, InvalidTransaction},
 };
 
@@ -130,6 +133,15 @@ where
 	}
 }
 
+impl<Address, Call, Signature, Extra> ExtrinsicMetadata
+	for UncheckedExtrinsic<Address, Call, Signature, Extra>
+		where
+			Extra: SignedExtension,
+{
+	const VERSION: u8 = TRANSACTION_VERSION;
+	type SignedExtensions = Extra;
+}
+
 /// A payload that has been signed for an unchecked extrinsics.
 ///
 /// Note that the payload that we sign to produce unchecked extrinsic signature
@@ -316,6 +328,7 @@ mod tests {
 	#[derive(Debug, Encode, Decode, Clone, Eq, PartialEq, Ord, PartialOrd)]
 	struct TestExtra;
 	impl SignedExtension for TestExtra {
+		const IDENTIFIER: &'static str = "TestExtra";
 		type AccountId = u64;
 		type Call = ();
 		type AdditionalSigned = ();