Perform extension packaging in extension-cli (#9549)

Release Notes:

- N/A

---------

Co-authored-by: Marshall <marshall@zed.dev>

crates/rpc/src/extension.rs

@@ -0,0 +1,10 @@
+use serde::{Deserialize, Serialize};
+
+#[derive(Serialize, Deserialize)]
+pub struct ExtensionApiManifest {
+    pub name: String,
+    pub version: String,
+    pub description: Option<String>,
+    pub authors: Vec<String>,
+    pub repository: String,
+}