ENH: Add global config manager

[arnavk23]

Reference Issues/PRs

Fixes #478

What does this implement/fix? Explain your changes.

This implements a global config manager for skbase as requested in issue #478.

It adds a global config system that allows setting config values globally, with priority order:

1. Local configs set via BaseObject.set_config
2. Global configs set via skbase.config.set_config
3. Defaults from class MRO

It also includes an extension point __skbase_get_config__ for downstream packages to provide their own global configs (e.g., for specific object types like forecasters in sktime).

The implementation includes:

• skbase.config module with get_config, set_config, reset_config, config_context
• Integration into BaseObject.get_config with the specified priority
• Comprehensive unit tests

Does your contribution introduce a new dependency? If yes, which one?

No

What should a reviewer concentrate their feedback on?

• The design of the global config priority order
• The extension mechanism for downstream packages
• Whether the API meets the requirements from the issue

Any other comments?

This is based on the draft in #149 but adjusted to meet the exact requirements from #478, including the priority order and extension for object types.

PR checklist

For all contributions

• I've reviewed the project documentation on contributing
• I've added myself to the list of contributors.
• The PR title starts with either [ENH], [CI/CD], [MNT], [DOC], or [BUG] indicating whether the PR topic is related to enhancement, CI/CD, maintenance, documentation, or a bug.

For code contributions

• Unit tests have been added covering code functionality
• Appropriate docstrings have been added (see documentation standards)
• New public functionality has been added to the API Reference

[codecov]

Codecov Report

❌ Patch coverage is 96.05263% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.57%. Comparing base (306958d) to head (a67ab9a).
⚠️ Report is 177 commits behind head on main.

Files with missing lines	Patch %	Lines
skbase/base/_base.py	72.72%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #491      +/-   ##
==========================================
- Coverage   85.07%   83.57%   -1.50%     
==========================================
  Files          45       55      +10     
  Lines        3015     3964     +949     
==========================================
+ Hits         2565     3313     +748     
- Misses        450      651     +201     

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[arnavk23]

@fkiraly whenever you are free, please take a look. Thanks!