Allow completing slash command arguments from extensions (#13240)

This PR extends the extension API with support for completing slash
command arguments for slash commands defined in extensions.

Release Notes:

- N/A

crates/extension_api/src/extension_api.rs

@@ -108,7 +108,16 @@ pub trait Extension: Send + Sync {
         None
     }
 
-    /// Runs the given slash command.
+    /// Returns the completions that should be shown when completing the provided slash command with the given query.
+    fn complete_slash_command_argument(
+        &self,
+        _command: SlashCommand,
+        _query: String,
+    ) -> Result<Vec<String>, String> {
+        Ok(Vec::new())
+    }
+
+    /// Returns the output from running the provided slash command.
     fn run_slash_command(
         &self,
         _command: SlashCommand,
@@ -225,6 +234,13 @@ impl wit::Guest for Component {
         Ok(labels)
     }
 
+    fn complete_slash_command_argument(
+        command: SlashCommand,
+        query: String,
+    ) -> Result<Vec<String>, String> {
+        extension().complete_slash_command_argument(command, query)
+    }
+
     fn run_slash_command(
         command: SlashCommand,
         argument: Option<String>,