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.

[lumtis]

FWIW with #2863 merged, the listBallot query would then list all pending ballots (instead of all ballots in the system) because completed ballots will be removed from the store.
We kept it aside because it was not seen as a direct priority and wanted to avoid the extensive upgrade migration of removing all ballots for now but if it is important we can look into have this one for the next release, I believe it would address the issue.