Install option for individual model providers instead of all

[EricThomson]

When installing jupyter-ai I'd like to be able to install specific classes of model providers without having to install all, e.g.:

uv pip install jupyter-ai[anthropic]

Instead of uv pip install jupyter-ai[all]

Note I'm assuming this doesn't already exist (I tried and got errors).

I'd be happy to help with this if possible (caveat I don't have much experience with this kind of thing so there would be a learning curve). It does seem like keeping track of all these optional dependencies could be a maintenance headache, so if the maintainers don't like this idea I get it. 😄

Related to #958

[dlqqq]

@EricThomson This is a very valid feature request, thank you for sharing this with us! Sorry this issue went undiscovered for some time.

I'll add this to our v3 milestone so we can target this for v3. 💪