extensions: add grpc_http1_reverse_bridge filter (#5315)

Adds a filter that allows converting a gRPC request into an HTTP/1.1
request with a custom content-type. Allows a vanilla HTTP/1.1 upstream
to handle incoming requests by reading and responding with protobuf
messages in binary octet format.

For now this shields the upstream from any gRPC association: the filter
removes the gRPC specific message prefix and manages the conversion of
the HTTP status code into grpc-status.

Signed-off-by: Snow Pettersen <snowp@squareup.com>

Mirrored from https://github.com/envoyproxy/envoy @ a97e138ffc58c13dc5be277b877c6a5949083023

envoy/config/filter/http/grpc_http1_reverse_bridge/v2alpha1/BUILD

@@ -0,0 +1,8 @@
+load("//bazel:api_build_system.bzl", "api_proto_library")
+
+licenses(["notice"])  # Apache 2
+
+api_proto_library(
+    name = "config",
+    srcs = ["config.proto"],
+)

envoy/config/filter/http/grpc_http1_reverse_bridge/v2alpha1/config.proto

@@ -0,0 +1,22 @@
+syntax = "proto3";
+
+package envoy.extensions.filter.http.grpc_http1_reverse_bridge.v2alpha1;
+option java_package = "io.envoyproxy.envoy.extensions.filter.http.grpc_http1_reverse_bridge.v2alpha1";
+option java_multiple_files = true;
+option go_package = "v2";
+
+import "validate/validate.proto";
+
+// [#protodoc-title: Extensions gRPC Http1 Reverse Bridge]
+// gRPC reverse bridge filter configuration
+message FilterConfig {
+  // The content-type to pass to the upstream when the gRPC bridge filter is applied.
+  // The filter will also validate that the upstream responds with the same content type.
+  string content_type = 1 [(validate.rules).string.min_bytes = 1];
+
+  // If true, Envoy will assume that the upstream doesn't understand gRPC frames and
+  // strip the gRPC frame from the request, and add it back in to the response. This will
+  // hide the gRPC semantics from the upstream, allowing it to receive and respond with a
+  // simple binary encoded protobuf.
+  bool withhold_grpc_frames = 2;
+}