Adding HTTP service support for Envoy external processing

[yanjunxiang-google]

Title: Adding HTTP service support for Envoy external processing

Description: Currently Envoy ext_proc filter can only talk to a gRPC service to do external processing. There is a need for ext_proc filter to be able to talk to a raw HTTP service to do the external processing.

[yanjunxiang-google]

It is planned to have multiple incremental PRs to support this feature:

1) API to add the API service 2) Adding ext_proc HTTP client 3) Refactor the ext_proc filter code to be able to support either gRPC client or HTTP client based on filter configuration. 4) Adding ext_proc HTTP service testing framework. 5) Adding integration test to test ext_proc filter <----> HTTP service end-to-end.

6) Update the API to describe how the HTTP messages will be constructed. 7) Adding HTTP request-ID and session affinity as in this comment:

https://github.com/envoyproxy/envoy/pull/35740#issuecomment-2307266324

x) TBD:

x.1) Do we need to support per-route override of http_service config, like here? https://github.com/envoyproxy/envoy/blob/6277f4601357550b0eba93fa3fd2a4154d63416b/api/envoy/extensions/filters/http/ext_proc/v3/ext_proc.proto#L372

This won't be considered in the MVP of ext_proc HTTP support.

x.2) Logging the HTTP ext_proc call, something like gRPC stream logging here: https://github.com/envoyproxy/envoy/blob/6277f4601357550b0eba93fa3fd2a4154d63416b/source/extensions/filters/http/ext_proc/ext_proc.cc#L898

x.3) Adding test for having the side stream server using HTTP1 protocol.

[soulxu]

cc @gbrail @stevenzzzz @tyxia @mattklein123 @htuch @yanavlasov

[yanjunxiang-google]

The processing flow works this way:

When Envoy needs to send call out messages to the side stream server, either header, body , or trailer, Envoy construct a ProcessingRequest proto message. 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. The HTTP server receives the "POST" message. Transcoding the body, i.e, a JSON text, into a ProcessingRequest message. The HTTP server then performs normal header/body/trailer mutation as existing, and put them into a ProcessingResponse proto message. The HTTP server then transcode the ProcessingResponse into JSON text, and sends back a 200 response with this JSON text as body. 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. The only difference is that in the transport layer, 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.