sdbus++: add type 'object_path' alias

The underlying dbus and sdbusplus type is 'object_path', but the
current sdbus++ type is 'path'.  Add an alias which is more obvious.

There are currently some users of 'path' is phosphor-dbus-interfaces,
so we cannot immediately remove 'path'.  Once this is merged we can
migrate them to 'object_path'.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Iee65fefe890b7bef351a39fa9fec403d3a086a78
diff --git a/test/server/Test.interface.yaml b/test/server/Test.interface.yaml
index 2b6c9f5..ac36b84 100644
--- a/test/server/Test.interface.yaml
+++ b/test/server/Test.interface.yaml
@@ -15,4 +15,4 @@
     - name: Countable
       type: size
     - name: ObjectPath
-      type: path
+      type: object_path