[Feature] Support for llama-cpp-python embedder and automatic selection when using llama as LLM

[AlessandroSpallina]

Description

Added:

• support for llama-cpp-python embedder served behind a REST API server
• automatic selection of the right embedder when llama 2 is used as LLM

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas