Add provider registry infrastructure and custom provider plugin example

[aksg87]

Description

Introduces a provider registry system enabling third-party providers to be dynamically registered and discovered through a plugin architecture. Users can now integrate custom LLM backends (Azure OpenAI, AWS Bedrock, custom inference servers) without modifying core LangExtract code.

Fixes #80, #67, #54, #49, #48, #53

Feature

How Has This Been Tested?

$ tox -e py310
$ pytest tests/registry_test.py -v
$ pytest tests/factory_test.py::FactoryTest -v
$ python examples/custom_provider_plugin/test_example_provider.py

Verified backward compatibility with existing providers and tested custom plugin with entry points.

Checklist:

• I have read and acknowledged Google's Open Source
  Code of conduct.
• I have read the
  Contributing
  page, and I either signed the Google
  Individual CLA
  or am covered by my company's
  Corporate CLA.
• I have discussed my proposed solution with code owners in the linked
  issue(s) and we have agreed upon the general approach.
• I have made any needed documentation changes, or noted in the linked
  issue(s) that documentation elsewhere needs updating.
• I have added tests, or I have ensured existing tests cover the changes
• I have followed
  Google's Python Style Guide
  and ran `pylint` over the affected code.

Key Changes

Provider Registry (`langextract/providers/registry.py`)

• Pattern-based registration with priority resolution
• Automatic discovery via Python entry points
• Lazy loading for performance

Factory Enhancements (`langextract/factory.py`)

• `ModelConfig` dataclass for structured configuration
• Explicit provider selection when patterns overlap
• Full backward compatibility maintained

Plugin Example (`examples/custom_provider_plugin/`)

• Complete working example with entry point configuration
• Shows how to create custom providers for any backend

Documentation

• Comprehensive provider system README with architecture diagrams
• Step-by-step plugin creation guide

Breaking Changes

No anticipated breakage - full backward compatibility maintained. Given significant internal changes to provider loading, issues should be reported if unexpected behavior is encountered.

[github-actions]

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