Separate type for unaggregated network attestations

[eserilev]

Description

Introduces a SingleAttestation type that helps differentiate between unaggregated and aggregated attestations during network processing>

Spec: ethereum/consensus-specs#3900

We'll probably want to implement this change in two or three stages:

• Add new SingleAttestation gossip topic. When receiving SingleAttestation objects over gossip, immediately convert to Attestation to reduce implementation complexity. This needs to be included before Electra. SingleAttestation implementation #6488
• Full implementation of SingleAttestsation. Don't convert it to a Attestation so that we reap the full benefits of the SingleAttestation changes. This can optionally be included before Electra. SingleAttestation refactor #6616
• Unify GossipAttestation and GossipSingleAttestation flows. This entails immediately converting Attestations received over gossip into SingleAttestations. This allows us to delete a bunch of code. This should probably only be included post Electra.