This is the first PR in support of
https://github.com/ledgerwatch/erigon/issues/5824.
The phase 0 sepc
https://github.com/ethereum/consensus-specs/blob/dev/specs/phase0/p2p-interface.md#global-topics
specifies 6 global pubsub topics that CL nodes need to handle.
This PR implements the `beacob_aggregate_and_proof` topic:
https://github.com/ethereum/consensus-specs/blob/dev/specs/phase0/p2p-interface.md#beacon_aggregate_and_proof.
The `AggregateAndProof` and `SignedAggregateAndProof` types are defined
here:
https://github.com/ethereum/consensus-specs/blob/dev/specs/phase0/validator.md#aggregateandproof.
I followed the implementation of `SignedBeaconBlockBellatrix`, which has
the following references:
1. cmd/lightclient/cltypes/types.go: defines the struct with relevant
SSZ annotations on the fields.
2. cmd/lightclient/cltypes/clone.go: this just returns a reference to an
empty object, so not super clear to me if it is necessary:
3. cmd/lightclient/rpc/common.go: this decodes gossip data, switching on
the type of gossip message that is received.
4. cmd/lightclient/sentinel/service/service.go: this listens on the
pubsub channel and notifies when a packet of the relevant type comes in.
5. cmd/lightclient/sentinel/pubsub.go: this defines the gossip topic
struct.
6. cmd/lightclient/lightclient/subscriber.go: this is the lightclient
interface for the incoming messages that come from the sentinel.
