Adds Snowbridge to Rococo runtime (#2522)

# Description

Adds Snowbridge to the Rococo bridge hub runtime. Includes config
changes required in Rococo asset hub.

---------

Co-authored-by: Alistair Singh <alistair.singh7@gmail.com>
Co-authored-by: ron <yrong1997@gmail.com>
Co-authored-by: Vincent Geddes <vincent.geddes@hey.com>
Co-authored-by: claravanstaden <Cats 4 life!>

cumulus/parachains/runtimes/bridge-hubs/common/src/digest_item.rs

@@ -0,0 +1,34 @@
+// Copyright (C) Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: Apache-2.0
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// 	http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//! Custom digest items
+
+use codec::{Decode, Encode};
+use sp_core::{RuntimeDebug, H256};
+use sp_runtime::generic::DigestItem;
+
+/// Custom header digest items, inserted as DigestItem::Other
+#[derive(Encode, Decode, Copy, Clone, Eq, PartialEq, RuntimeDebug)]
+pub enum CustomDigestItem {
+	#[codec(index = 0)]
+	/// Merkle root of outbound Snowbridge messages.
+	Snowbridge(H256),
+}
+
+/// Convert custom application digest item into a concrete digest item
+impl From<CustomDigestItem> for DigestItem {
+	fn from(val: CustomDigestItem) -> Self {
+		DigestItem::Other(val.encode())
+	}
+}