Add GetTicket grpc endpoint

[matheusd]

This call should receive the ticket hash and return the information as if it were a single element of a GetTickets call (ie, one instance of a GetTicketsResponse message).

The point of this is to be able to find the respective spender (either a vote or revocation) for an arbitrary ticket. The opposite (going from a vote/revocation to a ticket hash) is easy to do, given that you can just check the appropriate outpoint of the spender transaction.

[dnldd]

Will sort this out.