This PR implements functionality to store outbound message commitments
in the BEEFY MMR leaf extra field for cross-chain verification. A new
`pallet-outbound-commitment-store` has been introduced to facilitate
this process.
## Changes
- Added a new `pallet-outbound-commitment-store` pallet to capture and
store outbound message commitments
- Implemented the `CommitmentHandler` to receive commitments from the
outbound queue and store them in the new pallet
- Updated the `LeafExtraDataProvider` to include these commitments in
the BEEFY MMR leaf extra field
## Implementation Details
The process works as follows:
1. The outbound queue generates a commitment hash for messages
2. The commitment is stored in the `pallet-outbound-commitment-store`
via the `CommitmentHandler`
3. The `LeafExtraDataProvider` retrieves the latest commitment and
includes it in the BEEFY MMR leaf extra field
4. This commitment can then be verified by other chains using the BEEFY
light client
The new pallet provides the necessary functions to store and retrieve
these commitments.
