Add YandexGPT Encoder

[KirillKukharev]

Pull Request Description:

This pull request adds a new encoder class, YandexGPTEncoder, for generating text embeddings using the YandexGPT API. The encoder integrates with the semantic_router.

Changes Introduced:

1. New Encoder Class: YandexGPTEncoder for YandexGPT API integration.
2. Initialization: Requires YandexGPT API key and catalog ID, which can be provided directly or via environment variables.
3. Method Implementation:
   • _initialize_client for setting up the API client.
   • get_headers for preparing the request headers.
   • call for generating embeddings by making API requests.

[Alkons]

I suppose it should be yandexgpt instead of yandex-gpt

[Alkons]

lgtm