Add grpc_json_transcoder_filter to contour

[yangyy93]

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Please describe the problem you have [A clear, concise, description of the problem you are facing. What is the problem that feature X would solve for you?] envoy supports grpc_json_transcoder_filter to allow RESTful JSON API clients to send requests to Envoy through HTTP and proxy to the gRPC service. Does contour have any plans to support it in the future?

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[github-actions[bot]]

The Contour project currently lacks enough contributors to adequately respond to all Issues.

This bot triages Issues according to the following rules:

• After 60d of inactivity, lifecycle/stale is applied
• After 30d of inactivity since lifecycle/stale was applied, the Issue is closed

You can:

• Mark this Issue as fresh by commenting
• Close this Issue
• Offer to help out with triage

Please send feedback to the #contour channel in the Kubernetes Slack