[ext_proc] let ext_proc filter follow the grpc spec, wait for trailers before continue the filter chain upon last main stream end_stream=true event.

*Title*: ext_proc side grpc stream should follow grpc spec to wait for trailers to end a session.

*Description*:

In Grpc, the trailers carries the grpc-status header terminates a rpc stream. see https://github.com/grpc/grpc/blob/master/doc/PROTOCOL-HTTP2.md#responses for the spec and https://carlmastrangelo.com/blog/why-does-grpc-insist-on-trailers for why.

The current impl of closing a stream in ext_proc:
https://github.com/envoyproxy/envoy/blob/main/source/extensions/filters/http/ext_proc/client_impl.cc#L75-L78
Our current grpc flow in ext_proc filter ignores the trailers by closeStream and resetStream all together, the later resetStream call would signal the remote server a CANCEL, while clean the sidestream and ignore any possible trailers that might have been sent by ext_proc server.

Instead, for the last de/encode{Headers,data} call, ext_proc filter should fire a half-close (closeStream), and wait until the trailers come back, then call continueProcessing().

In PR #37083 we moved one step towards this goal: split half-close from reset(), but we would like to move the half-close earlier to the filter callbacks, before onDestroy gets called.

Noteworthy that there are special case in GRPC: Headers only response carries grpc-status in response headers, in which case we should proactively close the side stream as well.