feat!: add support for domains

[federicobond]

This PR

Adds support for domains with a minimalist implementation. Feedback is especially welcome around public APIs and backward compatibility issues.

Builds on top of #270

Related Issues

Refs #125

Follow-up Tasks

• Ensure the API is idiomatic and matches the spec guidelines
• Test backward compatibility with existing clients and providers

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (0ec2b69) 94.40% compared to head (0425a15) 94.52%.

Files	Patch %	Lines
openfeature/provider/registry.py	95.65%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #271      +/-   ##
==========================================
+ Coverage   94.40%   94.52%   +0.11%     
==========================================
  Files          17       18       +1     
  Lines         447      493      +46     
==========================================
+ Hits          422      466      +44     
- Misses         25       27       +2     

Flag	Coverage Δ
unittests	94.52% <96.92%> (+0.11%)	⬆️

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[gruebel]

Due to the argument renaming it becomes a breaking change and then you actually could completely get rid of _provider and initialize _providers like this

_providers: typing.Dict[typing.Optional[str], AbstractProvider] = {None: NoOpProvider()}

[federicobond]

I introduced a ProviderRegistry in the last commit similar to the ones in other SDKs because the code was becoming a bit convoluted.

[federicobond]

Rebased on top of latest main.