Snowbridge - Ethereum Client - Public storage items (#4501)

Changes the Ethereum client storage scope to public, so it can be set in
a migration.

When merged, we should backport to the all other release branches:

- [ ] release-crates-io-v1.7.0 - patch release the fellows BridgeHubs
runtimes https://github.com/paritytech/polkadot-sdk/pull/4504
- [ ] release-crates-io-v1.8.0 -
https://github.com/paritytech/polkadot-sdk/pull/4505
- [ ] release-crates-io-v1.9.0 -
https://github.com/paritytech/polkadot-sdk/pull/4506
- [ ] release-crates-io-v1.10.0 -
https://github.com/paritytech/polkadot-sdk/pull/4507
- [ ] release-crates-io-v1.11.0 -
https://github.com/paritytech/polkadot-sdk/pull/4508
- [ ] release-crates-io-v1.12.0 (commit soon)