[ENH] V1 → V2 API Migration - core structure

[geetu040]

Towards #1575

This PR sets up the core folder and file structure along with base scaffolding for the API v1 → v2 migration.

It includes:

• Skeleton for the HTTP client, backend, and API context
• Abstract resource interfaces and versioned stubs (*V1, *V2)
• Minimal wiring to allow future version switching and fallback support

No functional endpoints are migrated yet. This PR establishes a stable foundation for subsequent migration and refactor work.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 51.28983% with 321 lines in your changes missing coverage. Please review.
✅ Project coverage is 52.37%. Comparing base (8a5532f) to head (8de99b7).

Files with missing lines	Patch %	Lines
openml/_api/clients/http.py	23.00%	164 Missing ⚠️
openml/_api/resources/base/versions.py	31.16%	53 Missing ⚠️
openml/_api/resources/base/fallback.py	26.31%	28 Missing ⚠️
openml/_api/setup/builder.py	44.44%	25 Missing ⚠️
openml/_api/setup/backend.py	66.66%	19 Missing ⚠️
openml/_api/resources/base/base.py	57.14%	15 Missing ⚠️
openml/_config.py	80.64%	12 Missing ⚠️
openml/cli.py	0.00%	4 Missing ⚠️
openml/_api/clients/minio.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1576      +/-   ##
==========================================
- Coverage   52.73%   52.37%   -0.37%     
==========================================
  Files          37       61      +24     
  Lines        4399     5035     +636     
==========================================
+ Hits         2320     2637     +317     
- Misses       2079     2398     +319     

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[PGijsbers]

I guess this is the way we are doing this now, so I left some minor remarks.
But please in future work with smaller PRs, for example separately handling requests, fallbacks, caching, grouping resource APIs, and so on. I do think those could've been individual PRs that would replace existing code, so it's much clearer what replaces what, and not introduce a lot of untested or unused code into the codebase.