add an experimental 'flutter build web' call to the backend

[devoncarew]

Add an experimental 'flutter build web' call to the backend:

• add an experimental api/v2/_flutterBuild call to the backend
• it's given a source file and invokes flutter build web; this is currently slower than we'd want but I believe there's some room for improvement in the execution time of flutter build web
• the flutter web frontend has been updated to use this compile call for both dart and flutter snippets

Many of the changes here are multiple copies of the proto related files. With this change, the new front-end can now run flutter snippets.

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[devoncarew]

[johnpryan]

Is this still in the draft stage? Or would you prefer to merge it into main?

[devoncarew]

Is this still in the draft stage? Or would you prefer to merge it into main?

I'd prefer to merge it - it'll make it easier to iterate on (the restful API is named w/ an underscore - _flutterBuild - to help make sure we communicate 'do not use' to people).

[devoncarew]

@kevmoo has asked that we look into instead using the skiawasm backend (likely loaded from a CDN). I'll add a todo.