Enable setting all options via ENV vars

[vartec]

Allow setting all options via environment variables using LLM as the default prefix.

Note that for groups (subcommands) the prefix includes group name; LLM_{group:upper}.

Example:

$ LLM_CHAT_MODEL_ID=mistral-7b-instruct-v0 llm chat 
Chatting with mistral-7b-instruct-v0
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$ llm chat
Chatting with gpt-3.5-turbo
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$ export LLM_CHAT_MODEL_ID=mistral-7b-instruct-v0
$ llm chat
Chatting with mistral-7b-instruct-v0
⋮