Create method getApiV1BoxesUnspentByAddress with no-confirmed state

Current explorer doesn't have method to prevent double spent attempt.

> Use case from spectrum user:
> "I had to wait until the transactions were confirmed before I could make the next swap. before that, the next swap was using the same boxes from my wallet that were already spent in mempool. hence "double spent attempt" error when trying to submit."

Start by locating the existing v1 boxes-by-address API and its tests, since no file or test is named in the issue. Verify the behavior against the stated mempool double-spend scenario; done means the new getApiV1BoxesUnspentByAddress method exposes the requested no-confirmed state so a subsequent swap does not select already-spent boxes.

Written by the indexing model from the issue text.