test: add more tests for RequiredOptional

Rename the variables `whaction` and `action` to `resource_action` to
improve code-readability.

Naming a variable `what` makes it difficult to understand what it is
used for.

Rename it to `gitlab_resource` as that is what is being stored.

The Gitlab documentation talks about them being resources:
https://docs.gitlab.com/ee/api/api_resources.html

This will improve code readability.

Since ac1c619cae6481833f5df91862624bf0380fef67 we delete parent arg keys
from the args dict so this has been trying to access the wrong attribute.

Move the `validate_attrs` function to be inside the `RequiredOptional`
class. It makes sense for it to be part of the class as it is working
on data related to the class.

There is no reason to return an `int` as we can always return a `str`
version of the `int`

Change `EncodedId` to always return an `EncodedId`. This removes the
need to have `mypy` ignore the error raised.

add exclusive tuple to RequiredOptional data class to support for
mutually exclusive attributes

consolidate _check_missing_create_attrs and _check_missing_update_attrs
from mixins.py into _validate_attrs in utils.py

change _create_attrs in board list manager classes from
required=('label_ld',) to
exclusive=('label_id','asignee_id','milestone_id')

closes https://github.com/python-gitlab/python-gitlab/issues/1897