Update go mod and vendor in client dir to 0.28.0

client/vendor/github.com/google/gnostic-models/extensions/README.md

@@ -0,0 +1,13 @@
+# Extensions
+
+**Extension Support is experimental.**
+
+This directory contains support code for building Gnostic extensio handlers and
+associated examples.
+
+Extension handlers can be used to compile vendor or specification extensions
+into protocol buffer structures.
+
+Like plugins, extension handlers are built as separate executables. Extension
+bodies are written to extension handlers as serialized
+ExtensionHandlerRequests.