Fetch next credentials for search.

    :param search_handle: Search handle (created by prover_open_credentials_search)
    :param count: Count of records to fetch
    :return: credentials_json: List of credentials:
    [{
        "referent": string, // cred_id in the wallet
        "attrs": {"key1":"raw_value1", "key2":"raw_value2"},
        "schema_id": string,
        "cred_def_id": string,
        "rev_reg_id": Optional<string>,
        "cred_rev_id": Optional<string>
    }]
    NOTE: The list of length less than the requested count means credentials search iterator is completed.

async def prover_fetch_credentials(search_handle: int,
                                   count: int) -> str:
    """
    Fetch next credentials for search.

    :param search_handle: Search handle (created by prover_open_credentials_search)
    :param count: Count of records to fetch
    :return: credentials_json: List of credentials:
    [{
        "referent": string, // cred_id in the wallet
        "attrs": {"key1":"raw_value1", "key2":"raw_value2"},
        "schema_id": string,
        "cred_def_id": string,
        "rev_reg_id": Optional<string>,
        "cred_rev_id": Optional<string>
    }]
    NOTE: The list of length less than the requested count means credentials search iterator is completed.
    """

    logger = logging.getLogger(__name__)
    logger.debug("prover_fetch_credentials: >>> search_handle: %r, count: %r",
                 search_handle,
                 count)

    if not hasattr(prover_fetch_credentials, "cb"):
        logger.debug("prover_fetch_credentials: Creating callback")
        prover_fetch_credentials.cb = create_cb(CFUNCTYPE(None, c_int32, c_int32, c_char_p))

    c_search_handle = c_int32(search_handle)
    c_count = c_uint(count)

    credentials_json = await do_call('indy_prover_fetch_credentials',
                                     c_search_handle,
                                     c_count,
                                     prover_fetch_credentials.cb)

    res = credentials_json.decode()
    logger.debug("prover_fetch_credentials: <<< res: %r", res)
    return res