buildkitd: add grpc reflection

This lets you run:

```
docker run -d --rm --privileged -p 1234:1234 moby/buildkit:local --addr tcp://0.0.0.0:1234
grpcurl -plaintext localhost:1234 list
```

to determine what APIs a grpc server supports.

Signed-off-by: Nick Santos <[email protected]>

Author: nicks

cmd/buildkitd/main.go

@@ -68,6 +68,7 @@ import (
 	tracev1 "go.opentelemetry.io/proto/otlp/collector/trace/v1"
 	"golang.org/x/sync/errgroup"
 	"google.golang.org/grpc"
+	"google.golang.org/grpc/reflection"
 )
 
 func init() {
@@ -274,6 +275,7 @@ func main() {
 		defer controller.Close()
 
 		controller.Register(server)
+		reflection.Register(server)
 
 		ents := c.GlobalStringSlice("allow-insecure-entitlement")
 		if len(ents) > 0 {