add grpc-web gateways for REST access to Journal and Shard APIs

Use the `github.com/grpc-ecosystem/grpc-gateway` project to
automatically generate REST handlers for portions of the Journals
and Shards gRPC services.

Also implement a CORS wrapper with configurable allowed origins.
Together these changes allow Gazette brokers and consumers to be
directly queried by browsers in cross-origin contexts.

Note that REST handlers automatically forward `Authorization: Bearer`
tokens into their corresponding gRPC handlers, so authorization checks
"just work".

Makefile

@@ -20,7 +20,9 @@ ci-release-gazette-examples-targets = \
 # Targets of protobufs which must be compiled.
 protobuf-targets = \
 	./broker/protocol/protocol.pb.go \
+	./broker/protocol/protocol.pb.gw.go \
 	./consumer/protocol/protocol.pb.go \
+	./consumer/protocol/protocol.pb.gw.go \
 	./consumer/recoverylog/recorded_op.pb.go \
 	./examples/word-count/word_count.pb.go
 

broker/protocol/protocol_gateway.yaml

@@ -0,0 +1,11 @@
+type: google.api.Service
+config_version: 3
+
+http:
+  rules:
+    - selector: protocol.Journal.List
+      post: /v1/journals/list
+      body: "*"
+    - selector: protocol.Journal.Read
+      post: /v1/journals/read
+      body: "*"

cmd/gazette/main.go

@@ -8,6 +8,8 @@ import (
 	"syscall"
 	"time"
 
+	"github.com/gogo/gateway"
+	"github.com/grpc-ecosystem/grpc-gateway/runtime"
 	"github.com/jessevdk/go-flags"
 	log "github.com/sirupsen/logrus"
 	"go.gazette.dev/core/allocator"
@@ -116,6 +118,13 @@ func (cmdServe) Execute(args []string) error {
 		signalCh = make(chan os.Signal, 1)
 	)
 	pb.RegisterJournalServer(srv.GRPCServer, pb.NewVerifiedJournalServer(service, verifier))
+
+	var mux *runtime.ServeMux = runtime.NewServeMux(
+		runtime.WithMarshalerOption(runtime.MIMEWildcard, new(gateway.JSONPb)),
+		runtime.WithProtoErrorHandler(runtime.DefaultHTTPProtoErrorHandler),
+	)
+	pb.RegisterJournalHandler(tasks.Context(), mux, srv.GRPCLoopback)
+	srv.HTTPMux.Handle("/v1/", Config.Broker.CORSWrapper(mux))
 	srv.HTTPMux.Handle("/", http_gateway.NewGateway(pb.NewRoutedJournalClient(lo, pb.NoopDispatchRouter{})))
 	ks.WatchApplyDelay = Config.Broker.WatchDelay