[GREEN] Implement route and slot plugin contribution support

[ksamaschke]

Phase

GREEN

## Deliverable
Implement the minimum plugin runtime changes needed to satisfy the route and slot contribution tests.

## Dependencies
- Parent work item: #218.

## Completion evidence
- Implementation notes for the updated plugin runtime.

• Evidence that the RED tests now pass.

  Acceptance criteria

  • Plugin UI contributions work against the desktop shell model.

• Tab-centric contribution is no longer the target architecture.

  Parent: Work Item: Migrate plugin contributions from tabs to route and slot APIs #218