Review and update problem-report endpoints

[wip-abramson]

The problem-report endpoint is used by present-proof and issue-credential and maybe others.

Currently the proofs API writes it like this

    async def send_problem_report(self, pres_ex_id, request):
        return await self.admin_POST(f"{self.base_url}/records/{pres_ex_id}/problem-report", json_data=request)

This is confusing because it should not be request.

Request is actually just:

{
  "explain_ltxt": "string"
}

Suggest we update request to explanation: str. And build the json object within the function.