fix: add readme.com extension for showing "Bearer" in auth

As `grpc-gateway` only supports OpenAPI 2.x, we were relying on the
field description to specify `Bearer` is part of the API key.

common/openapi/v1beta/api_config.conf

@@ -16,6 +16,10 @@
         in: IN_HEADER;
         name: "Authorization";
         description: "Enter the token with the `Bearer ` prefix, e.g. `Bearer abcde12345`";
+        extensions: {
+          key: "x-default";
+          value {string_value: "Bearer instill_sk_***"}
+        }
       }
     }
   }