Create a high level VoteOnBallot function

[lumtis]

Describe the Issue
Context: https://github.com/zeta-chain/node/pull/1511/files/76e2198cc6aa3ed3dfbfd0992456bcebda74e2d3#r1500968292 and https://github.com/zeta-chain/node/pull/1511/files/76e2198cc6aa3ed3dfbfd0992456bcebda74e2d3#r1500963921

We have several message implying voting on Ballot:

• inbound vote
• outbound vote
• tss vote
• blame vote
• block header vote

Each implement common logic like fetching the ballot, adding vote, checking if finalized.

I would make more sense to create a high level function that can factorize the common logic.

Expected Outcome

• Implement a function like VoteOnBallot in the observer module
• Add unit test to this function
• Change logic of the messages listed above to include this function