PoS Simplify PublicKey field names in P2P messages

[tholonious]

Simple PR that renames fields in the MsgValidatorVote and MsgValidatorTimeout message types. It renames:

• ValidatorPublicKey -> PublicKey
• ValidatorVotingPublicKey -> VotingPublicKey

The "Validator" prefix in the field names isn't needed in the field names since it's already contextualized in the message type.

[tholonious]

Current dependencies on/for this PR:

• feature/pos-messaging-schema
  • PR PoS Add P2P vote and timeout messages #555
    • PR PoS Add QC fields to block header schema #558
      • PR PoS Implement Bitset data structure #564
        • PR PoS Add Bitset to messaging schema #565
          • PR PoS Simplify PublicKey field names in P2P messages #566 👈
            • PR PoS Add BLS public key and signature to BlockProducerInfo #567
              • PR PoS Add ProposedInView field to block header #568
                • PR PoS Compress uint64 fields in MsgDeSoHeader version 2 #569

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