Governance: Create Proposals and Sign Off workflow

[SebastianBor]

Implemented instructions:

• CreateProposal
• AddSignatory
• RemoveSignatory
• SignOffProposal

Design changes:

1. VoterRecord renamed to TokenOwnerRecord
   Since Governing Token deposit is required to create a Proposal I reused VoterRecord account and its token_deposit_amount to validate eligibility to create Proposals. By doing so the account become more than just a VoterRecord and I renamed it accordingly.

2. vote_authority renamed to governance_delegate
   For the same reasons as 1) the authority is not just for voting any longer. Also since granting the authority doesn't take voting or proposal creation rights from the token_owner I renamed it to governance_delegate to better express its meaning.
   SetVoteAuthority instruction was changed accordingly to SetGovernanceDelegate

3. The change in 1) and 2) eliminated the need for Proposal Admin Mint/Token because TokenOwnerRecord becomes Proposal owner and it's Governance Authority (Token Owner or Delegate) automatically acquires admin rights to edit Proposal.

4. I also removed Proposal Signatory Mint/Token and replaced it with SignatoryRecord account to track signatories. The main reason for this was the spl token scheme made it impossible for Proposal owner to remove Signatory because once minted the token would be owned by the Signatory and not by Proposal owner.
   However I also think it made it simpler and simplified Proposal accounts

5. I removed GoverningTokenType and replaced it with the corresponding Governing Token Mint. I think it not only fits nicer this way into the api but also could allow us to easily support N mints per Realm (which was voiced during api reviews) in future versions.

[joncinque]

Looking really good, just some nits and a few questions

[joncinque]

one nit, which you can take or leave