Convert CredentialsNDBProperty into a JsonProperty

[dhermes]

See TODO (being removed in #434).

Turn this into a JsonProperty and overhaul the Credentials and subclass mechanics to use new_from_dict, to_dict, from_dict, etc.

[theacodes]

Thank you for creating this issue, however, this project is deprecatedand we will only be addressing critical security issues. You can read moreabout this deprecation here.

If you need support or help using this library, we recommend that you ask yourquestion on StackOverflow.

If you still think this issue is relevant and should be addressed, pleasecomment and let us know!