Update API to return 'source_id' for users (#29718)

Using the API, a user's _source_id_ can be set in the _CreateUserOption_
model, but the field is not returned in the _User_ model.

This PR updates the _User_ model to include the field _source_id_ (The
ID of the Authentication Source).

templates/swagger/v1_json.tmpl

@@ -24260,6 +24260,12 @@
           "type": "boolean",
           "x-go-name": "Restricted"
         },
+        "source_id": {
+          "description": "The ID of the user's Authentication Source",
+          "type": "integer",
+          "format": "int64",
+          "x-go-name": "SourceID"
+        },
         "starred_repos_count": {
           "type": "integer",
           "format": "int64",