Use rust-analyzer's flycheck as source of cargo diagnostics (#29779)

Follow-up of https://github.com/zed-industries/zed/pull/29706

Instead of doing `cargo check` manually, use rust-analyzer's flycheck:
at the cost of more sophisticated check command configuration, we keep
much less code in Zed, and get a proper progress report.

User-facing UI does not change except `diagnostics_fetch_command` and
`env` settings removed from the diagnostics settings.

Release Notes:

- N/A

crates/proto/src/proto.rs

@@ -171,6 +171,9 @@ messages!(
     (LspExtSwitchSourceHeaderResponse, Background),
     (LspExtGoToParentModule, Background),
     (LspExtGoToParentModuleResponse, Background),
+    (LspExtCancelFlycheck, Background),
+    (LspExtRunFlycheck, Background),
+    (LspExtClearFlycheck, Background),
     (MarkNotificationRead, Foreground),
     (MoveChannel, Foreground),
     (MultiLspQuery, Background),
@@ -425,6 +428,9 @@ request_messages!(
     (SynchronizeContexts, SynchronizeContextsResponse),
     (LspExtSwitchSourceHeader, LspExtSwitchSourceHeaderResponse),
     (LspExtGoToParentModule, LspExtGoToParentModuleResponse),
+    (LspExtCancelFlycheck, Ack),
+    (LspExtRunFlycheck, Ack),
+    (LspExtClearFlycheck, Ack),
     (AddWorktree, AddWorktreeResponse),
     (ShutdownRemoteServer, Ack),
     (RemoveWorktree, Ack),
@@ -548,6 +554,9 @@ entity_messages!(
     SynchronizeContexts,
     LspExtSwitchSourceHeader,
     LspExtGoToParentModule,
+    LspExtCancelFlycheck,
+    LspExtRunFlycheck,
+    LspExtClearFlycheck,
     LanguageServerLog,
     Toast,
     HideToast,