Issue/add discovery resource support

Description

Currently the mitogen base classes only work for purgeable resources and crud handlers. That makes them unusable for the discovery resource. This MR makes the mitogen interface a bit more generic (and keep the original implementations for backward compatibility).

Reviewers list

• reviewer 1

Self Check:

Strike through any lines that are not applicable (~~line~~) then check the box

• Changelog entry and version bump inmanta module release --dev [--major|--minor|--patch] [--changelog-message "<your_changelog_message>"]
• Code is clear and sufficiently documented (classes, methods and function docs, document important assumptions, link to reference materials)
• Sufficient test cases (reproduces the bug/tests the requested feature)
• Correct, in line with design
• Type annotations
• Issue is in the review column
• MR is assigned to first reviewer