git blame: Display GitHub avatars in blame tooltips, if available (#10767)

Release Notes:

- Added GitHub avatars to tooltips that appear when hovering over a `git
blame` entry (either inline or in the blame gutter).

Demo:



https://github.com/zed-industries/zed/assets/1185253/295c5aee-3a4e-46aa-812d-495439d8840d

crates/git/src/git.rs

@@ -10,6 +10,7 @@ pub use lazy_static::lazy_static;
 pub mod blame;
 pub mod commit;
 pub mod diff;
+pub mod hosting_provider;
 pub mod permalink;
 pub mod repository;