Feat/issue 497 implement custom embedding model

[Pouyanpi]

Summary

This PR implements EmbeddingProviderRegistry that extends the singleton base Registry class. It resolves #497.

Key Changes:

• Implement base Registry class. This class provides a solid foundation for registering and managing different components in NeMo Guardrails.
• Implement singleton. This ensures that we only ever have a single instance of the Registry class in our system.
• Implement EmbeddingProviderRegistry. It validates models based on whether they have encode and encode_async methods (duck typing).
• Implement register_embedding_provider. This allows users to register their own embedding providers.

TODO:

Add documentation