Add a schema to extensions, to prevent installing extensions on too old of a Zed version (#9599)

Release Notes:

- N/A

---------

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

crates/rpc/src/extension.rs

@@ -7,4 +7,5 @@ pub struct ExtensionApiManifest {
     pub description: Option<String>,
     pub authors: Vec<String>,
     pub repository: String,
+    pub schema_version: Option<i32>,
 }