c/st: self_test_result rpc: move {start,end}_time to end of layout

Effectively turning the original change into an append and obviating
the need for a compat version bump.

It's not immediately clear why the struct was laid out this way in
the first place. A reasonable guess is some balance of:

- The original ordering makes good sense for the aggregate in-memory
- It's an rpc format and not intended for on-disk persistence

Still, the compat version change means that if a self test goes off
during an upgrade across the compat boundary, that rpc will fail.
Adjusting the layout via envelope::serde_fields means that we can
maintain the nice field order semantics in the aggregate while
ensuring that the diff looks like a field append from serde's view.

src/v/cluster/self_test_rpc_types.h

@@ -241,7 +241,7 @@ struct cloudcheck_opts
 
 struct self_test_result
   : serde::
-      envelope<self_test_result, serde::version<1>, serde::compat_version<1>> {
+      envelope<self_test_result, serde::version<1>, serde::compat_version<0>> {
     double p50{0};
     double p90{0};
     double p99{0};
@@ -289,6 +289,27 @@ struct self_test_result
           r.error ? *r.error : "<no_value>");
         return o;
     }
+
+    auto serde_fields() {
+        return std::tie(
+          p50,
+          p90,
+          p99,
+          p999,
+          max,
+          rps,
+          bps,
+          timeouts,
+          test_id,
+          name,
+          info,
+          test_type,
+          duration,
+          warning,
+          error,
+          start_time,
+          end_time);
+    }
 };
 
 struct empty_request