Allow filling co-authors in the git panel's commit input (#23329)

https://github.com/user-attachments/assets/78db908e-cfe5-4803-b0dc-4f33bc457840


* starts to extract usernames out of `users/` GitHub API responses, and
pass those along with e-mails in the collab sessions as part of the
`User` data

* adjusts various prefill and seed test methods so that the new data can
be retrieved from GitHub properly

* if there's an active call, where guests have write permissions and
e-mails, allow to trigger `FillCoAuthors` action in the context of the
git panel, that will fill in `co-authored-by:` lines, using e-mail and
names (or GitHub handle names if name is absent)

* the action tries to not duplicate such entries, if any are present
already, and adds those below the rest of the commit input's text

Concerns:

* users with write permissions and no e-mails will be silently omitted
— adding odd entries that try to indicate this or raising pop-ups is
very intrusive (maybe, we can add `#`-prefixed comments?), logging seems
pointless

* it's not clear whether the data prefill will run properly on the
existing users — seems tolerable now, as it seems that we get e-mails
properly already, so we'll see GitHub handles instead of names in the
worst case. This can be prefilled better later.

* e-mails and names for a particular project may be not what the user
wants.
E.g. my `.gitconfig` has
```
[user]
    email = mail4score@gmail.com

# .....snip

[includeif "gitdir:**/work/zed/**/.git"]
    path = ~/.gitconfig.work
```

and that one has

```
[user]
    email = kirill@zed.dev
```

while my GitHub profile is configured so, that `mail4score@gmail.com` is
the public, commit e-mail.

So, when I'm a participant in a Zed session, wrong e-mail will be
picked.
The problem is, it's impossible for a host to get remote's collaborator
git metadata for a particular project, as that might not even exist on
disk for the client.

Seems that we might want to add some "project git URL <-> user name and
email" mapping in the settings(?).
The design of this is not very clear, so the PR concentrates on the
basics for now.

When https://github.com/zed-industries/zed/pull/23308 lands, most of the
issues can be solved by collaborators manually, before committing.

Release Notes:

- N/A

crates/call/src/cross_platform/participant.rs

@@ -40,6 +40,15 @@ pub struct LocalParticipant {
     pub role: proto::ChannelRole,
 }
 
+impl LocalParticipant {
+    pub fn can_write(&self) -> bool {
+        matches!(
+            self.role,
+            proto::ChannelRole::Admin | proto::ChannelRole::Member
+        )
+    }
+}
+
 pub struct RemoteParticipant {
     pub user: Arc<User>,
     pub peer_id: proto::PeerId,
@@ -57,4 +66,11 @@ impl RemoteParticipant {
     pub fn has_video_tracks(&self) -> bool {
         !self.video_tracks.is_empty()
     }
+
+    pub fn can_write(&self) -> bool {
+        matches!(
+            self.role,
+            proto::ChannelRole::Admin | proto::ChannelRole::Member
+        )
+    }
 }

crates/call/src/macos/participant.rs

@@ -39,6 +39,15 @@ pub struct LocalParticipant {
     pub role: proto::ChannelRole,
 }
 
+impl LocalParticipant {
+    pub fn can_write(&self) -> bool {
+        matches!(
+            self.role,
+            proto::ChannelRole::Admin | proto::ChannelRole::Member
+        )
+    }
+}
+
 #[derive(Clone, Debug)]
 pub struct RemoteParticipant {
     pub user: Arc<User>,
@@ -57,4 +66,11 @@ impl RemoteParticipant {
     pub fn has_video_tracks(&self) -> bool {
         !self.video_tracks.is_empty()
     }
+
+    pub fn can_write(&self) -> bool {
+        matches!(
+            self.role,
+            proto::ChannelRole::Admin | proto::ChannelRole::Member
+        )
+    }
 }