[ENH] Migration: set up core/base 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 85.36585% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.42%. Comparing base (bd8ae77) to head (58e9175).

Files with missing lines	Patch %	Lines
openml/_api/resources/tasks.py	86.95%	6 Missing ⚠️
openml/_api/runtime/core.py	81.48%	5 Missing ⚠️
openml/_api/http/client.py	75.00%	4 Missing ⚠️
openml/_api/resources/datasets.py	75.00%	2 Missing ⚠️
openml/_api/__init__.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1576      +/-   ##
==========================================
- Coverage   79.14%   73.42%   -5.73%     
==========================================
  Files          36       46      +10     
  Lines        4320     4440     +120     
==========================================
- Hits         3419     3260     -159     
- Misses        901     1180     +279     

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