sdbus++: async: server: generate get-property fn

Add binding generation for get-property functions.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I60d44b25d718373acfd581faf158f45f5b5430e9
diff --git a/example/calculator-aserver.cpp b/example/calculator-aserver.cpp
index 220cda1..536ee25 100644
--- a/example/calculator-aserver.cpp
+++ b/example/calculator-aserver.cpp
@@ -13,6 +13,16 @@
         ctx.spawn(startup());
     }
 
+    auto get_property(owner_t) const
+    {
+        return "asdf";
+    }
+
+    auto get_property(last_result_t) const
+    {
+        return 42;
+    }
+
   private:
     auto startup() -> sdbusplus::async::task<>
     {