Private NFTs: ownership proofs

[aguycalled]

We need to implement the ability to prove ownership of specific private NFTs.

An NFT owner should be able to create a cryptographic proof which can be verified to confirm its correctness.

A valid cryptographic proof could be a message "OWNERSHIP_PROOF_NFT"||OUTPOINT||TOKENID signed with the NFT's spending key where:

• "OWNERSHIP_PROOF_NFT" is a string literal
• || is the concatenation operator
• OUTPOINT is the current UTXO of the NFT we are proving ownership of as a string in the format txid:vout
• TOKENID is the serialisation of the NFT Identifier as per the Navcoin-core rules (32 bytes uint256 hash concatenated with uint64 integer)

A validator would receive this proof together with the TOKENID and would check the latest OUTPOINT of the NFT from a full node. Then will verify the signature with the spending key of that outpoint to confirm or reject the ownership claim.

Changes needed in navcoin-core:

• Implement an index which tracks the last UTXO of each NFT navcoin-core#919
• Show last UTXO of an NFT through RPC command navcoin-core#920

Changes needed in electrumx:

• Support for the RPC changes related to showing last UTXO of an NFT

Changes needed in navcoin-js:

• Support creation of NFT ownership proofs navcoin-js#15
• Support verification of NFT ownership proofs navcoin-js#16