Add Snowbridge initialize migration on Westend

[yrong]

Description

Fix #5074 which missed the runtime migration to initialize channels of the bridge.

[claravanstaden]

This is not really a backport, so perhaps change the title to "Add Snowbridge initialize migration on Westend", otherwise the title is misleading.

[yrong]

https://github.com/paritytech/polkadot-sdk/actions/runs/10923847845/job/30321556177?pr=5747

check-runtime-migration(bridge-hub-rococo) fails though seems nothing to do with this PR. Tested with try-runtime-cli and it work as expected as follows:

➜  polkadot-sdk git:(snowbridge-westend-migration) ✗ RUST_LOG=ethereum_system try-runtime --runtime ./target/release/wbuild/bridge-hub-westend-runtime/bridge_hub_westend_runtime.compact.compressed.wasm on-runtime-upgrade --checks=pre-and-post live --uri wss://westend-bridge-hub-rpc.polkadot.io:443
✅ Found 6698 keys (3.23s)
[00:00:17] ✅ Downloaded key values 391.0557/s [====================================================================================================================================================] 6698/6698 (0s)
✅ Inserted keys into DB (0.03s)
[2024-09-19T01:40:40Z INFO  ethereum_system::migration] Agents and channels not initialized. Initialization will run.
[2024-09-19T01:40:40Z INFO  ethereum_system::migration] Ethereum system initialized.
[2024-09-19T01:40:40Z INFO  ethereum_system::migration] Ethereum system initialized.
[2024-09-19T01:40:40Z INFO  ethereum_system::migration] Agents and channels are initialized. Initialization will not run.
[2024-09-19T01:40:40Z INFO  ethereum_system::migration] Ethereum system already initialized. Skipping.

@acatangiu Is there anything required left for this PR?

[acatangiu]

@yrong just some spurious network issues leading to some CI tests failing, restarted those CI jobs, should be auto-merged when they finish