feat: Add GigaChat Encoder

[KirillKukharev]

Summary

This pull request introduces the GigaChatEncoder class, a new encoder for generating embeddings using the GigaChat API. The implementation includes initialization, embedding generation, and error handling functionalities.

Changes

• Added GigaChatEncoder class to the project.
• The encoder class initializes with GigaChat client credentials, name, scope.
• The call method generates embeddings for a list of documents.
• Includes error handling and validation for required attributes.
• Add unit tests and docs

Implementation Details

Initialization:

• The init method initializes the encoder with optional parameters: name, auth_data, scope, and score_threshold.
• The method sets default values using EncoderDefault.GIGACHAT if name is not provided.
• It validates the auth_data and scope parameters, raising ValueError if they are None.
• Initializes the GigaChat client with provided credentials and scope.

Embedding Generation:

• The call method takes a list of documents and generates embeddings.
• Validates that the GigaChat client is initialized.
• Calls the embeddings method of the GigaChat client to generate embeddings.
• Extracts and returns the embeddings as a list of lists.
• Raises a ValueError if the client is not initialized or if the GigaChat call fails.