[meta] 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

• feat: allow registering all in-memory table types via create_table #6593

P1

• feat(descheming): add catalog kwarg to table accessing methods #9000 (needs to wait until 9.1)
• chore: deprecate register functionality  #8735 (PR open refactor: deprecate register api #8863 blocked on feat: allow registering all in-memory table types via create_table #6593)
• feat: Ability to Distinguish between Table and Views #8382 (PR open feat(ddl): distinguish views from tables while listing them #8864 needs review)
• feat(parquet): figure out convention for multi-file parquet writing #8584
• bug(pyspark): export functions don't take param or limit #8898
• refactor(polars): more accurate type mapping for timestamps #8479 (PR open feat(polars): add more accurate type mapping for timestamps #8954 under review)
• refactor: standardize use of keyword/positional-only arguments #9125

P2

• feat: support passing pa.RecordBatch in con.insert/con.create_table #8133
• feat: allow creating temporary views in create_view #8727
• feat: add a to_json() output method #8866

[gforsyth]

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

[ncclementi]

Closing in favor of #9638