Emit log on Runtime Code change. (#9580)

* Emit digest item on Runtime Code changes.

* Add tests.

* cargo +nightly fmt --all

* Rename.

* Add comment.

* Move generic parameter to the trait.

* cargo +nightly fmt --all

* Elaborate in doc.

* Revert to RuntimeUpdated name

* cargo +nightly fmt --all

* Rename to RuntimeEnvironmentUpdated

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

@@ -118,6 +118,14 @@ pub enum DigestItem<Hash> {
 
 	/// Some other thing. Unsupported and experimental.
 	Other(Vec<u8>),
+
+	/// An indication for the light clients that the runtime execution
+	/// environment is updated.
+	///
+	/// Currently this is triggered when:
+	/// 1. Runtime code blob is changed or
+	/// 2. `heap_pages` value is changed.
+	RuntimeEnvironmentUpdated,
 }
 
 /// Available changes trie signals.
@@ -184,6 +192,8 @@ pub enum DigestItemRef<'a, Hash: 'a> {
 	ChangesTrieSignal(&'a ChangesTrieSignal),
 	/// Any 'non-system' digest item, opaque to the native code.
 	Other(&'a Vec<u8>),
+	/// Runtime code or heap pages updated.
+	RuntimeEnvironmentUpdated,
 }
 
 /// Type of the digest item. Used to gain explicit control over `DigestItem` encoding
@@ -199,6 +209,7 @@ pub enum DigestItemType {
 	Seal = 5,
 	PreRuntime = 6,
 	ChangesTrieSignal = 7,
+	RuntimeEnvironmentUpdated = 8,
 }
 
 /// Type of a digest item that contains raw data; this also names the consensus engine ID where
@@ -225,6 +236,7 @@ impl<Hash> DigestItem<Hash> {
 			Self::Seal(ref v, ref s) => DigestItemRef::Seal(v, s),
 			Self::ChangesTrieSignal(ref s) => DigestItemRef::ChangesTrieSignal(s),
 			Self::Other(ref v) => DigestItemRef::Other(v),
+			Self::RuntimeEnvironmentUpdated => DigestItemRef::RuntimeEnvironmentUpdated,
 		}
 	}
 
@@ -322,6 +334,7 @@ impl<Hash: Decode> Decode for DigestItem<Hash> {
 			DigestItemType::ChangesTrieSignal =>
 				Ok(Self::ChangesTrieSignal(Decode::decode(input)?)),
 			DigestItemType::Other => Ok(Self::Other(Decode::decode(input)?)),
+			DigestItemType::RuntimeEnvironmentUpdated => Ok(Self::RuntimeEnvironmentUpdated),
 		}
 	}
 }
@@ -457,6 +470,9 @@ impl<'a, Hash: Encode> Encode for DigestItemRef<'a, Hash> {
 				DigestItemType::Other.encode_to(&mut v);
 				val.encode_to(&mut v);
 			},
+			Self::RuntimeEnvironmentUpdated => {
+				DigestItemType::RuntimeEnvironmentUpdated.encode_to(&mut v);
+			},
 		}
 
 		v