[EPIC] Ibis backend public API stability

[lostmygithubaccount]

Acceptance criteria: This quarter, we will have a precise plan and initial work toward a stable public backend interface.

The purpose of this work is to set a stable foundation for new backends to be added, particularly be external contributors/organizations. We should work toward allowing external backends to be fully contributed, maintained, and owned in external repositories.

[ncclementi]

After re-triaging/scoping some issues these issues belong to this epic, leaving them here for reference:

P0

• [x] https://github.com/ibis-project/ibis/issues/6593

P1

• [ ] https://github.com/ibis-project/ibis/issues/9000 (needs to wait until 9.1)
• [x] https://github.com/ibis-project/ibis/issues/8735 (PR open https://github.com/ibis-project/ibis/pull/8863 blocked on #6593)
• [ ] https://github.com/ibis-project/ibis/issues/8382 (PR open https://github.com/ibis-project/ibis/pull/8864 needs review)
• [ ] https://github.com/ibis-project/ibis/issues/8584
• [x] https://github.com/ibis-project/ibis/issues/8898
• [ ] https://github.com/ibis-project/ibis/issues/8479 (PR open https://github.com/ibis-project/ibis/pull/8954 under review)
• [ ] https://github.com/ibis-project/ibis/issues/9125

P2

• [ ] https://github.com/ibis-project/ibis/issues/8133
• [ ] https://github.com/ibis-project/ibis/issues/8727
• [ ] https://github.com/ibis-project/ibis/issues/8866

[gforsyth]

I've opened a PR with a POC for lightly enforcing signature compatibility between backends in #9383