feat(statement-distribution) handle `Backed` signal

## Issue summary

- [ ] Implement `handle_backed_candidate_message` https://github.com/paritytech/polkadot-sdk/blob/7725890d114ea420e2d946b4d9e0a823d20cf0d8/polkadot/node/network/statement-distribution/src/v2/mod.rs#L2824C21-L2824C52

Its necessary to have [`StatementStore`](https://github.com/paritytech/polkadot-sdk/blob/16e877be9afef58e3376d46ea96067aecd5b0dad/polkadot/node/network/statement-distribution/src/v2/statement_store.rs#L59C12-L59C26) implemented, it is used to store statements signed under the same relay-parent.

We need it to use within `local_knowledge_filter`, basically fills a `StatementFilter` to be used in the grid topology with all statements we are already aware of.

- [x] Implement `StatementFilter` https://github.com/paritytech/polkadot-sdk/blob/16e877be9afef58e3376d46ea96067aecd5b0dad/polkadot/node/network/protocol/src/lib.rs#L630

- [x] Implement `StatementStore` https://github.com/paritytech/polkadot-sdk/blob/16e877be9afef58e3376d46ea96067aecd5b0dad/polkadot/node/network/statement-distribution/src/v2/statement_store.rs#L59C12-L59C26

## Other information and links

- https://github.com/paritytech/polkadot-sdk/blob/aaf0443591b134a0da217d575161872796e75059/polkadot/node/network/statement-distribution/src/lib.rs#L434