Blueprints: Add a git Resource type

[adamziel]

#1764 enables browsing and fetching any git files from public repos. Let's create a new git Blueprint Resource type to make that a first-class primitive. It should accept a repository URL, ref, and some form of paths. Challenge: Resources can only expose files today, but with git it would be useful to also expose directories.

Goal: The import modal should produce a working Blueprint. It should not directly pull any files into Playground.

It should also support arbitrary path mappings like the export modal: