Epic: Join Order Enumeration

Related: [Discussion: API for Join Access Path and Join Order Selection](https://github.com/apache/datafusion/issues/17718)

This Epic Ticket outlines high-level components and examples for us to explore and define APIs for Join Access Path and Join Order Selection.

This ticket aims to provide a high-level but comprehensive view, including:
- [A Join Graph for TPC-H Query 5](https://github.com/apache/datafusion/issues/18250)
- [Join Order Enumeration for Q5 using abstract Join Ranking and Plan Pruning](https://github.com/apache/datafusion/issues/18251)
- [A customizable Join Ranking example](https://github.com/apache/datafusion/issues/18257) for API design, considering some interesting properties
- [Some Plan Pruning Strategies](https://github.com/apache/datafusion/issues/18258)
- [Do we want a simple or traditional cost model](https://github.com/apache/datafusion/issues/18259)?

The goal is to spark deeper discussion and help break this epic into focused, actionable tickets.

No files, tests, or entry points are named. Start with the linked discussion and the child issues 18250, 18251, 18257, 18258, and 18259 to understand the proposed join graph, enumeration, ranking, pruning, and cost-model questions. Done means the high-level exploration has been turned into focused, actionable tickets with defined APIs.

Written by the indexing model from the issue text.