Validate that the attestation bitlist size matches the committee size

[ajsutton]

PR Description

When creating an indexed attestation we need to ensure that the aggregation bits are the same size as the committee. There are four places this can occur:

• When a BeaconState contains invalid PendingAttestation waiting processing at the end of the epoch. This will never occur "naturally" but could occur if an invalid start state is provided.
• When an attestation is received via gossip. We need to create the indexed attestation as part of validating it.
• As part of running fork choice rule on received attestations. Partial validation was in place here - if the bitlist was too short the attestation was rejected, but if it was too large it was processed.
• When an attestation is received as part of a block. Validation is already in place here.

An explicit precondition check has been added in get_attesting_indices so that it's now impossible to create an indexed attestation if the bitlist size is incorrect. All of the above cases now will reject the attestation.

The gossip attestation validator also performs an explicit check so it can return a standard INVALID result instead of throwing an IllegalStateException. Fork choice now catches InvalidArgumentException when creating the indexed attestation instead of IndexOutOfBoundsException (which is what was thrown if the bitlist was too short). The other cases already expect and handle the precondition check exception.

Fixed Issue(s)

fixes #1685
covers at least part of #1686

[cemozerr]

LGTM