leanEthereum/leanSpec#67

There are newly added/modifed containers in this spec PR. At this moment, we won't use ProposerAttestationData.

My suggestion here:

• Vote -> Rename AttestationData
• Add Attestation and AggregatedAttestation
• SignedVote -> Rename to SignedAttestation and fix message type as Attestation
• Fix existing attestation.rs as well.

I guess it would be mostly renaming and making the existing codebase compile.