Separate type for aggregated committee attestation

[ensi321]

Description

Introducing separate container for aggregated attestation in a committee. The new container replaces the unnecessary committee bits with committee index. It also has 63x shorter max length than Attestation on aggregation_bits which yields max size going from 16.23kb to 0.48kb. This ensures aggregates with 63 committees worth of zeros in aggregation bits with not be deserialized.

The current Attestation represents aggregated attestation both on-chain and in a committee. Having a dedicated container for committee attestation improves readability and removes the need to maintain a one-bit-set committee bit list.

[ppopth]

Suggested change

	The proposer should run the following function to construct an on chain final aggregate form a list of committee attestations with equal `AttestationData`:
	The proposer should run the following function to construct an on-chain final aggregate from a list of committee attestations with equal `AttestationData`:

[ppopth]

Making it the same as in "Construct attestation" would be easier. Reordering this to be the second one as in "Construct attestation" would even be better.

Suggested change

	- Set `attestation.committee_index = committee_index`, where `committee_index` is the `committee_index` in each individual attestation.
	- Set `attestation.committee_index` to the index associated with the validator's committee.

[ppopth]

I think these two sections are trivial and wordy and unrelated to the change, right? I would say they are not needed.

Suggested change

	#### Aggregate signature
	Set `aggregate_attestation.signature = aggregate_signature` where `aggregate_signature` is obtained from:
	```python
	def get_aggregate_signature(attestations: Sequence[CommitteeAttestation]) -> BLSSignature:
	signatures = [attestation.signature for attestation in attestations]
	return bls.Aggregate(signatures)
	```
	### Broadcast aggregate
	`get_aggregate_and_proof` is modified to accept `CommitteeAttestation` for `aggregate`.
	```python
	def get_aggregate_and_proof(state: BeaconState,
	aggregator_index: ValidatorIndex,
	committee_attestation: CommitteeAttestation,
	privkey: int) -> AggregateAndProof:
	return AggregateAndProof(
	aggregator_index=aggregator_index,
	aggregate=committee_attestation,
	selection_proof=get_slot_signature(state, aggregate.data.slot, privkey),
	)
	```

[ppopth]

Referencing #3900 (comment) and #3787 which have kind of similar ideas.

[ppopth]

Looks good to me after all