Initial LLM integration

[sgreenbury]

Closes #91.

This PR:

• Adds popgetter-llm crate (LLM crate) providing search with text embeddings and DataRequestSpec with langchain
• Integrates the LLM crate with the CLI (optionally installable with llm feature)

Next steps:

• Write instructions for: init, query, ...

[sgreenbury]

Currently tests passing locally but not on the runner - not clear the reason. Fixed by 79cb65e (compilation memory size appears to have been causing process to be killed).