feat: Improve proof api

Problem to solve

At the moment the proof api supports:

1. Issue proof with connection
2. Accept with connection
3. List received proof request that are not accepted.

We are working on implementing oob proofs.

There is request from mohammed.hashem via the email for supporting get single proof request.

Do we need any additional improvements over the API besides, get by id and decline request?

@kalin.canov @boyan.tsolov

Related issue: #34 (closed)