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/collab/src/db.rs

@@ -731,6 +731,7 @@ pub struct NewExtensionVersion {
     pub description: String,
     pub authors: Vec<String>,
     pub repository: String,
+    pub schema_version: i32,
     pub published_at: PrimitiveDateTime,
 }