Use blockchain events to get notified for block imports

## Issue summary

- Currently in Gossamer we have the BlockchainEvents that is a new way to get notifications about client blockchains events: `block imported` and `block finalized`

## Implementation details

-

## Other information and links

- https://github.com/paritytech/polkadot-sdk/blob/21fbd6b59d37fd18a00d0ec4b6f72dc376d63010/polkadot/node/service/src/builder/mod.rs#L637

- https://github.com/paritytech/polkadot-sdk/blob/e2d3da6128bbd6a00e2036503fc2ed3e9c3c1b80/polkadot/node/overseer/src/lib.rs#L322

## Acceptance criteria

[] Add AC's here..
[] Regression testing (if applicable)
[] New code is 60% covered with unit tests