(WIP) NFT Proxy Voting

[austinmilt]

This PR (when finished) will add support for proxy voting using fungible tokens to boost NFT voting power in the nft-voter plugin. To do this, it adds three instructions and program methods that mimic the way that the base spl-governance program handles voting tokens:

1. Create a fungible token holding account per registered NFT, to which fungible token owners can deposit tokens to boost a given NFT's voting power.
2. Deposit fungible tokens into NFT's holding account, which creates a receipt/record of the total amount deposited, for the owner to withdraw from the holding account when it is safe to do so.
3. Withdraw fungible tokens from the NFT's holding account supplying the deposit receipt from above, when it is safe to do so.

Changes (TBD)