Add vendor dependencies as part git repo

vendor/github.com/google/gnostic/extensions/README.md

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+# 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.