Get json cryptonym (including current verification key) for input (agent) DID from ledger.

        Raise BadLedgerTxn on failure.

        :param did: DID of cryptonym to fetch
        :return: cryptonym json

async def get_nym(self, did: str) -> str:
        """
        Get json cryptonym (including current verification key) for input (agent) DID from ledger.

        Raise BadLedgerTxn on failure.

        :param did: DID of cryptonym to fetch
        :return: cryptonym json
        """

        LOGGER.debug('_BaseAgent.get_nym >>> did: %s', did)

        rv = json.dumps({})
        get_nym_req = await ledger.build_get_nym_request(self.did, did)
        resp_json = await self._submit(get_nym_req)

        data_json = (json.loads(resp_json))['result']['data']  # it's double-encoded on the ledger
        if data_json:
            rv = data_json

        LOGGER.debug('_BaseAgent.get_nym <<< %s', rv)
        return rv