Add v2 attestation aggregation interface

[dB2510]

Problem to be solved

We are planning to add attestation aggregation duty which will require changes in go-eth2-client package which we use as types and interfaces for calling a beacon client. Before making changes in go-eth2-client repo we need to test attestation aggregation on our side.

Proposed solution

• Create an experimental sub-package as eth2exp in eth2util package where we will add all the types required to support aggregation duties until go-eth2-client supports them.
• Add v2 BeaconCommitteeSubscriptionsSubmitter interface to eth2util/eth2exp package. Add a TODO: should be removed once it is added to go-eth2-client. Refer design doc for interface and structs.
• Add the above interface to eth2wrap and beaconmock.
• Add SubmitBeaconCommitteeSubscriptionsFunc to Mock struct in testutil/beaconmock.go which will be called by SubmitBeaconCommitteeSubscriptions method of Mock. Add following Option func to testutil/options.go:

func WithAttestationAggregation(aggregators map[eth2p0.Slot]eth2p0.ValidatorIndex) Option {
	return func(mock *Mock) {
		mock.SubmitBeaconCommitteeSubscriptionsFunc = func(ctx context.Context, subscriptions []*apiv2.BeaconCommitteeSubscription) ([]apiv2.BeaconCommitteeSubscriptionResponse, error) {
		}
	}
}

which will return response based on the aggregators map for the given slot and validator index.

• Add unit tests