Update SSZ containers regarding vote/attestation

[syjn99]

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 ValidatorAttestation
• SignedVote -> Rename to SignedValidatorAttestation and fix message type as ValidatorAttestation
• Fix existing attestation.rs as well.

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

[syjn99]

@varun-doshi You might start here, without adding ProposerAttestationData