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/ui/src/components/icon.rs

@@ -94,6 +94,7 @@ pub enum IconName {
     PageDown,
     PageUp,
     Pencil,
+    Person,
     Play,
     Plus,
     Public,
@@ -193,6 +194,7 @@ impl IconName {
             IconName::Option => "icons/option.svg",
             IconName::PageDown => "icons/page_down.svg",
             IconName::PageUp => "icons/page_up.svg",
+            IconName::Person => "icons/person.svg",
             IconName::Pencil => "icons/pencil.svg",
             IconName::Play => "icons/play.svg",
             IconName::Plus => "icons/plus.svg",