feat: Install LiteLLM by default

[ludwiktrammer]

Currently ragbits and ragbits-core are installed without installing LiteLLM (which requires explicitely installing ragbits[litellm].

Motivation

1. Ragbits requires LiteLLM to do almost anything useful (besides more advanced use cases of running local LLMs)
2. After feat(llms): default values for default_llm_factories #154, Ragbits comes pre configured with default LLM values which use LiteLLM. This currently leads to Ragbits not being able to use its own defaults by default ;)

Steps

• ragbits-core should list LiteLLM as its direct dependency.
• Conditional imports of litellm should be changes to regular imports
• Make LiteLLM available from the root ragbits.core.llms module
• Test that LiteLLM can imported without full path by LLM.subclass_from_config by adding test similar to tests.unit.embeddings.test_from_config.test_subclass_from_config_default_path to tests.unit.llms.test_from_config
• All places that mention ragbits[litellm] in the documentation (e.g., Quickstart 1) should be adjusted accordingly

[mhordynski]

I totally agree! I'll go ahead and add it to 0.5.0 Milestone. It looks like a great improvement for a small effort.