handling different string formats in RESTler

Any value can be used as the "format" value for string types in an OpenAPI specification. To better support domain-specific formats, RESTler could handle this in a generic way by generating a new dictionary entry, which the user could then populate. If empty, it would be treated as a restler_fuzzable_string.

For example: restler_custom_payload_format("date") restler_custom_payload_format("uri")

and in the dictionary:

"restler_custom_payload_format": {
  "date": ["2021-10-02"],
  "uri": ["https://example.com"]
}