Add customizable embedding model support for OpenAI API

[rajandangi]

This pull request enables the specification of embedding models in the OpenAI settings. Changes include updates to configuration files and the embedding component.

Changes Made

• Modified settings-openai.yaml to accept embedding_model under the llm section.
• Updated settings.yaml to define embedding_model for the OpenAI service.
• Altered embedding_component.py in the private_gpt to initialize OpenAIEmbedding with a user-defined model.
• Added embedding_model field in private_gpt/settings/settings.py with a default value and description.

This enhancement allows for flexible selection of OpenAI embedding models, enhancing adaptability to various use cases.

[imartinez]

Covers the same functionality as https://github.com/zylon-ai/private-gpt/pull/1920.