Update notebook magics to work with Gemini

[markmcd]

Adds some aliases for the commands too:

• %%palm still works, for backwards compat
• %%llm as a generic magic
• %%gemini as an analogue to %%palm

Motivation

Need to work with the new API and new models.

Type of change

Choose one: New feature

Checklist

• I have performed a self-review of my code.
• I have added detailed comments to my code where applicable.
• I have verified that my change does not break existing code.
• My PR is based on the latest changes of the main branch (if unsure, please run git pull --rebase upstream main).
• I am familiar with the Google Style Guide for the language I have coded in.
• I have read through the Contributing Guide and signed the Contributor License Agreement.

[MarkDaoust]

I broke the presubmit tests for this. Have you run the tests manually?

[MarkDaoust]

We should probably merge #141 first.

[markmcd]

Yeah I have run them manually but will rebase properly & ensure they pass in CI first.

[MarkDaoust]

Rebased onto main.