Plugin Discovery Mechanism and Aggregation

Description

Implement the dynamic discovery mechanism (evaluating Python entry_points vs. directory convention) to find definitions in other installed modules. The goal is to collect all paths and pass them to the Loader from #410 in a single batch.

Acceptance Criteria

• Select and implement the discovery mechanism (Python entry_points is strongly recommended to align with Open edX standards).
• Implement the Collection phase: aggregate all discovered definitions in memory BEFORE passing them to the validator to allow for cross-cutting validation.
• Refactor openedx-authz: modify the core module so it stops using the static path (from #410 and starts being discovered by this exact same dynamic mechanism, ensuring core and contributions use the identical pipeline.
• Create a "fake" test module (test_plugin) simulating an external contribution to confirm it is automatically discovered without modifying openedx-authz code.

Proposed ADR: https://github.com/openedx/openedx-authz/pull/423