Notify about attestations from the pending att queue

[rkapka]

What type of PR is this?

Other

What does this PR do? Why is it needed?

In the single attestation PR we moved sending the attestation through the notifier's feed to after all validations have been performed (see beacon-chain/sync/validate_beacon_attestation.go). This makes sense because we shouldn't notify other parts of the codebase that we received an attestation when that attestation is rejected or ignored. There is one scenario when this is an issue, though. If we receive an attestation which we can't process immediately, we save it to the pending att queue and exit early from the validation function, which means we don't send it through the feed. This is fine, but we need to send it later when we reprocess it at a later time in the pending att queue.

Acknowledgements

• I have read CONTRIBUTING.md.
• I have included a uniquely named changelog fragment file.
• I have added a description to this PR with sufficient context for reviewers to understand this PR.