Ext proc http functionality support

[yanjunxiang-google]

Risk Level: low
Testing: n/a
Docs Changes: n/a
Release Notes: inline
Fixes:

Description:
This is to address the issue: #35488, i.e, integrate the ext_proc HTTP client to ext_proc filter. With this PR, the basic functionalities to have Envoy ext_proc filter talk to a HTTP server using HTTP messages are accomplished.

This is the follow up of PR: #35676

[yanjunxiang-google]

This PR adds the support of sending ext_proc call out messages to the HTTP side stream server. The proto messages: ProcessingRequest/ProcessingResponse are transcoded into JSON text and encoded in the body of a HTTP message.

The processing flow works this way:

1. When Envoy needs to send call out messages to the side stream server, either header, body , or trailer, Envoy construct a ProcessingRequest proto message.
2. This ProcessingRequest message is transcoded into a JSON text. This JSON text is then added as a body to a HTTP "POST" message. This HTTP message is sent to the side stream HTTP server.
3. The HTTP server receives the "POST" message. Transcoding the body, i.e, a JSON text, into a ProcessingRequest message.
4. The HTTP server then performs normal header/body/trailer mutation as existing, and put them into a ProcessingResponse proto message.
5. The HTTP server then transcode the ProcessingResponse into JSON text, and sends back a 200 response with this JSON text as body.
6. After receives the 200 response, Envoy then convert the body from JSON text back to a ProcessingResponse proto message. Then continue processing this ProcessingResponse.

Overall, the ext_proc gRPC and HTTP share identical state machine and logic. The only difference is in the message sending mechanism. One using gRPC, and the ProcessingRequest/ProcessingResponse messages are sent as proto messages. The other sends messages using HTTP, with the ProcessingRequest/ProcessingResponse proto messages are transcoded into JSON text, and encoded as the body in the HTTP messages.

[yanjunxiang-google]

/retest

[yanjunxiang-google]

/docs

[repokitteh-read-only]

Docs for this Pull Request will be rendered here:

https://storage.googleapis.com/envoy-pr/35740/docs/index.html

The docs are (re-)rendered each time the CI envoy-presubmit (precheck docs) job completes.

🐱

Caused by: a #35740 (comment) was created by @yanjunxiang-google.

see: more, trace.

[yanjunxiang-google]

@envoyproxy/api-shepherds PTAL and approve

[adisuissa]

Thanks!
Let's make this unimplemented-hide and make it under work-in-progress so this can be tested properly and finalized later.

[adisuissa]

/lgtm api

[yanjunxiang-google]

/retest

[yanjunxiang-google]

Kind Ping!

[yanavlasov]

/wait