AUTH/PAY APIs using unmaintained Flask-RESTplus dependency

[sutherlanda]

What is the problem?

• Flask-RESTPlus is a dependency in both the auth- and pay- APIs (and other registries APIs). The maintainers have declared they will no longer be supporting RESTPlus and have started a new project based off a fork: RESTx. RESTPlus is also no longer compatible with the latest version of Werkzeug as of Feb 6 (and they only released a hotfix for RESTx, not RESTPlus). We should switch over to the new RESTx framework.

References:
noirbizarre/flask-restplus#770
noirbizarre/flask-restplus#777
https://github.com/python-restx/flask-restx

What is the impact?

• Since RESTPlus is no longer supported, it will be more difficult to maintain the project going forward (The broken Werkzeug dependency being an initial example of this).

Proposed solution

• Update the APIs to use Flask-RESTx

cc: @pksumesh @thorwolpert