Relay-parent digest logs for parachains (#2552)

* add digest item for relay-parent to primitives

* add a relay-parent-storage-root digest as a workaround

* more docs

* deposit log in pallet-parachain-system

* even more docs

* fix duplicate imports after botched mertge

* fix hyperlinks in docs

* clean up match

Co-authored-by: Bastian Köcher <git@kchr.de>

* improve docs

* fix typo

* add number to the digest item

---------

Co-authored-by: Bastian Köcher <git@kchr.de>

cumulus/pallets/parachain-system/src/tests.rs

@@ -1006,3 +1006,18 @@ fn upgrade_version_checks_should_work() {
 		});
 	}
 }
+
+#[test]
+fn deposits_relay_parent_storage_root() {
+	BlockTests::new().add_with_post_test(
+		123,
+		|| {},
+		|| {
+			let digest = System::digest();
+			assert!(cumulus_primitives_core::rpsr_digest::extract_relay_parent_storage_root(
+				&digest
+			)
+			.is_some());
+		},
+	);
+}