Submit all slot attestations at once

[rkapka]

What type of PR is this?

Other

What does this PR do? Why is it needed?

https://ethereum.github.io/beacon-APIs/?urls.primaryName=dev#/Beacon/submitPoolAttestations Beacon API endpoint allows submitting multiple attestations in a single request. We currently do not take advantage of this and submit one attestation at a time. To allow this the PR extracts a GetAttestationData function from SubmitAttestation so that we can build up a slice of attestation data values and pass it all at once for submission.