google_ai: Add Gemini 2.0 Flash support (#22665)

Release Notes:

- Added support for Google's Gemini 2.0 Flash experimental model.

Note:

Weirdly enough the model is slow on small talk responses like 'hi' (in
my tests) but very fast on things that need more tokens like 'write me a
snake game in python'. Likely an API problem.

TESTED ONLY ON WINDOWS! Would test further but don't have Linux
installed and don't have an Mac. Will likely work everywhere.

Why?:

I think Gemini 2.0 Flash is incredibly good model at coding and
following instructions. I think it would be nice to have it in the
editor. I did as minimal changes as possible while adding the model and
streaming validation. I think it's worth merging the commits as they
bring good improvements.

---------

Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>

crates/language_model/src/model/cloud_model.rs

@@ -88,6 +88,7 @@ impl CloudModel {
             Self::Google(model) => match model {
                 google_ai::Model::Gemini15Pro
                 | google_ai::Model::Gemini15Flash
+                | google_ai::Model::Gemini20Flash
                 | google_ai::Model::Custom { .. } => {
                     LanguageModelAvailability::RequiresPlan(Plan::ZedPro)
                 }