Rework Avatar indicator to be more general-purpose (#4073)

This PR reworks the way we add indicators to `Avatar`s to make them more
general-purpose.

Previously we had logic specific to the availability indicator embedded
in the `Avatar` component, which made it unwieldy to repurpose for
something else.

Now the `indicator` is just a slot that we can put anything into.

Release Notes:

- N/A

crates/ui/src/components/stories/avatar.rs

@@ -1,8 +1,8 @@
 use gpui::Render;
 use story::Story;
 
-use crate::prelude::*;
 use crate::Avatar;
+use crate::{prelude::*, Availability, AvatarAvailabilityIndicator};
 
 pub struct AvatarStory;
 
@@ -19,11 +19,11 @@ impl Render for AvatarStory {
             ))
             .child(
                 Avatar::new("https://avatars.githubusercontent.com/u/326587?v=4")
-                    .availability_indicator(true),
+                    .indicator(AvatarAvailabilityIndicator::new(Availability::Free)),
             )
             .child(
                 Avatar::new("https://avatars.githubusercontent.com/u/326587?v=4")
-                    .availability_indicator(false),
+                    .indicator(AvatarAvailabilityIndicator::new(Availability::Busy)),
             )
     }
 }