Add a new class to handle Multipart requests

[lbalmaceda]

Changes

Implements a new class to handle multipart requests. This is required before implementing the "import users" Job endpoint.

These multipart requests are only allowed for HTTP methods other than GET. Some of the shared logic between CustomRequest and the introduced MultipartRequest has been moved up to a class named ExtendedBaseRequest (as it's adding things on top of BaseRequest). This shared logic class is package-private!

To review:

• Name of the introduced request class: MultipartRequest.
• The existence (or not) of the FormDataRequest interface.
• The exceptions that should be declared as thrown on the abstract methods for creating the request body and parsing the response body.

Testing

Check the commit history!! First, I added the new request class. Then I added tests for that. Then I added a few helper classes for the tests. Lastly, I refactored and extracted the common parts to the shared logic class.

Please, do check for public API breaking changes. There shouldn't be any, but is always good to have another pair of eyes 👀

• This change adds test coverage
• This change has been tested on the latest version of the platform/language or why not

Checklist

• I have read the Auth0 general contribution guidelines
• I have read the Auth0 Code of Conduct
• All existing and new tests complete without errors

[jimmyjames]

Regarding names, I think MultipartRequest makes sense. I think we might be able to do better than ExtendedBaseRequest - I know it's package private so not a huge deal, but as you add some JavaDoc to it maybe a more descriptive name will become obvious.

I also think the interface is fine, just add JavaDoc please ✍️

Regarding the exception types, let's connect later on this and discuss 👍

In general it looks good though, just a couple general points:

• I know our existing test coverage provides confidence in not introducing any breaking changes, but it would be good to double check and verify that there are no unintended breaking changes to any of the impacted public APIs, especially any changes to method signatures/exception types.
• JavaDocs 😄