chore: embed Config into Source (#1864)

To keep a persistent backend storage for configuration, we will have to
keep a single source of truth. This involves supporting bi-directional
conversion between Config and Source.


This PR make the following changes:
* Embed Config in Source
* Add `ToConfig()` to extract Config from Source.

internal/server/server_test.go

@@ -137,8 +137,10 @@ func TestUpdateServer(t *testing.T) {
 
 	newSources := map[string]sources.Source{
 		"example-source": &alloydbpg.Source{
-			Name: "example-alloydb-source",
-			Kind: "alloydb-postgres",
+			Config: alloydbpg.Config{
+				Name: "example-alloydb-source",
+				Kind: "alloydb-postgres",
+			},
 		},
 	}
 	newAuth := map[string]auth.AuthService{"example-auth": nil}