Add a mechanism to output all incomplete inbound ballots

[CryptoFewka]

Is your feature request related to a problem? Please describe.
It is difficult to determine if all observers are observing newly integrated chains. In order to determine the observer voting status for an inbound transaction, we currently would have to collect the inbound ballot identifier from the logs of an observer, and then call /zeta-chain/observer/has_voted/{ballot_identifier}.

Describe the solution you'd like
Ideally there would be a way to list open ballots as well as who has voted. Presumably there already is a mechanism for this inside of the protocol, but it is not exposed currently.

Describe alternatives you've considered
Vote messages could be pulled from the blockchain itself, but would require the use of an indexer and a custom solution.

Motivation and Context
We would like to monitor inbound CCTX observation, and would like an API endpoint to output inbound CCTX observation, or a mechanism to determine the ballot identifier that can then be used with /zeta-chain/observer/has_voted/{ballot_identifier} to monitor voting.