SignedContributionAndProof broadcast

After building a `SignedContributionAndProof` in #837, the sync aggregator needs to send it over the `sync_committee_contribution_and_proof` after the second interval of the slot is finished.

refs spec: [link](https://github.com/lambdaclass/lambda_ethereum_consensus/blob/main/docs/specs/validator.md#broadcast-sync-committee-contribution)