This will also fix other use cases where an integer is passed in to
MultipartEncoder.

Added unit tests to show it works.

Closes: #2506

I found the tuple of three values confusing. So instead use a
dataclass to return the three values. It is still confusing but a
little bit less so.

Also add some unit tests

The purpose of this change is to track API changes described in
https://github.com/python-gitlab/python-gitlab/blob/main/docs/api-levels.rst,
for example, for package versioning and breaking change announcements
in case of protocol changes.

This is MVP implementation to be used by #2435.