Loosen Restrictions for Certain P2P Attestation Validation Conditions

[rauljordan]

What type of PR is this?

Other

What does this PR do? Why is it needed?

This PR includes two important changes required by https://github.com/ethereum/eth2.0-specs/pull/1801/files#diff-2e616bac0161143d97e596d333a60561R279, which specifically:

• Validate an aggregate attestation as long as the attester's index is in the beacon committee. Previously, we would have a stricter check which would enforce the attester's index is in the attesting indices
• Change the check for whether an unaggregated attestation is unaggregated to len(attestationutil.AttestingIndices(att.AggregationBits, committee)) != 1. Previously, we would just check if att.AggregationBits.Count() != 1

Which issues(s) does this PR fix?

Part of #5935

[codecov]

Codecov Report

Merging #5962 into v0.12 will increase coverage by 0.02%.
The diff coverage is 38.46%.

@@            Coverage Diff             @@
##            v0.12    #5962      +/-   ##
==========================================
+ Coverage   59.32%   59.34%   +0.02%     
==========================================
  Files         319      319              
  Lines       27060    27069       +9     
==========================================
+ Hits        16052    16064      +12     
+ Misses       8797     8794       -3     
  Partials     2211     2211