Title: Support TCP Proxy over HTTP/2 POST

Description:
#13185 has original context. With #13548, Envoy could supports the encapping TCP over HTTP/2 POST with a custom TCP upstream.

Based on our further investigation, we notice that in addition to a custom TCP upstream for encapping for the client side envoy, we also need to a symmetric decapping filter on the server side envoy.

We would like to contribute to Envoy upstream because we think it will benefit other Envoy users as well, as many edge proxies don't have good support for HTTP/2 CONNECT (for example, Google edge frontend).

If this proposal is accepted, I would like some guidance of

1. whether the custom upstream should be an independent class or it can be an option to the existing tcp upstream?

2. which directory is the proper place for adding the custom upstream (if needed) and the new filter?