Disallow shell history embedding unless explicitly configured

[guywaldman]

This PR:

• Adds a new general.access_to_shell_history configuration flag for explicitly allowing access to shell history
• Adds a new search.allow_remote_llm configuration flag for explicitly allowing access to shell history to non-local LLM providers
• Adds a new search.shell_history configuration for specifying a custom path to a shell history file (added for testing, but may be optionally useful for users none the less)
• Adds a new search.index_dir configuration for specifying a custom path to a directory which will contain the indexing data and metadata (added for testing, but may be optionally useful for users none the less)
• Adds and improves on E2E test coverage
• Fixes an issue where embeddings wouldn't work for OpenAI since embedding dimensions were provided and were throwing an error (see Remove dims for OpenAI orch#13)
• Various fixes & enhancements

Addresses #30