Google Gemini Plugin

[gogwilt]

Adds a plugin for Google Vertex AI (specifically Gemini).

[gogwilt]

Google API oddities:

• At least with inlineData, you can only interact with Gemini Pro Vision for a single "turn." If you have a chat conversation of length > 1, and any of those messages have an image, you get a 400 error.
• The API spec says that you can only use inlineData for a single image. Otherwise, you have to upload to GCS. I'm guessing that this restriction is even tighter, based on the bullet point above.

For reference, here's a graph that causes Gemini Pro Vision to fail.

[gogwilt]

Remote debugging seems to work. I couldn't get local npm modules to work, so the test was pretty janky. Will test once this is published.

Google auth seems to work. At a minimum, setting the app default credentials json file to a bogus file made it stop working... Probably worth asking people whether they're able to get it working once we publish this.

GCS storage is still not done, but I think it would be best to add this in a subsequent PR. For now, Gemini is fully usable in text-only mode, and vision is usable with some restrictions.

[gogwilt]

@abrenneke - I'd love to get your review on this! I think it's worth getting a release out for it.

[abrenneke]

Looks good to me! Thanks!