fix(server): handle gRPC-Web requests over HTTP2

[maxbrunet]

After much testing, I found out the following:

HTTP1.1 works: ✔️

$ curl --http1.1 127.0.0.1:7070/api/grpc.health.v1.Health/Check \
  -H 'Content-Type: application/grpc-web-text' \
  -H 'Accept: application/grpc-web-text' \
  -d 'AAAAAAA='
AAAAAAIIAQ==gAAAABBncnBjLXN0YXR1czogMA0K

H2C (HTTP2 upgraded from HTTP1.1) works: ✔️

$ curl --http2 127.0.0.1:7070/api/grpc.health.v1.Health/Check \
  -H 'Content-Type: application/grpc-web-text' \
  -H 'Accept: application/grpc-web-text' \
  -d 'AAAAAAA='
AAAAAAIIAQ==gAAAABBncnBjLXN0YXR1czogMA0K

HTTP2 (without upgrade from HTTP1.1) does not work: ❌

$ curl --http2-prior-knowledge 127.0.0.1:7070/api/grpc.health.v1.Health/Check \
  -H 'Content-Type: application/grpc-web-text' \
  -H 'Accept: application/grpc-web-text' \
  -d 'AAAAAAA='
invalid gRPC request content-type "application/grpc-web-text"

This patch fixes it: ✔️

$ curl --http2-prior-knowledge 127.0.0.1:7070/api/grpc.health.v1.Health/Check \
  -H 'Content-Type: application/grpc-web-text' \
  -H 'Accept: application/grpc-web-text' \
  -d 'AAAAAAA='
AAAAAAIIAQ==gAAAABBncnBjLXN0YXR1czogMA0K

gRPC-Web requests can use HTTP2, and most modern browsers actually use it if the host supports it.

This latest case happens if the reverse proxy (e.g. Envoy) forwards HTTP2 to HTTP2. Until now, we had been using NGINX in the demo which downgrades the request to HTTP1.1, so the issue had not been surfaced.

This should unblock advanced use cases with mixed client protocols (HTTP1.1/HTTP2/H2C, gRPC/gRPC-Web).