chore: add 3.4.0 schema (note: identical to 3.3.1)

Author: james-garner-canonical

juju/client/SCHEMAS.md

@@ -23,3 +23,6 @@ Please see docs/CONTRIBUTING.md for the process for updating this file.
 3.3.5 (identical to 3.3.1)
 3.3.6 (identical to 3.3.1)
 3.3.7 (identical to 3.3.1)
+
+# 3.4
+3.4.0 (note: identical to 3.3.1)