Allow gateway to enable gRPC reflection

[fbac]

Add gateway configuration and builder support to enable gRPC server reflection for the Gateway service

• Add Reflection field to config.GatewayConfig with struct tags group:"Reflection Options" and namespace:"reflection" in gateway.go
• Add gateway.GatewayServiceBuilder.WithReflection to set b.config.Reflection.Enable in builder.go
• Update gateway.GatewayServiceBuilder.buildGatewayService to conditionally register gRPC reflection via reflection.Register(grpcServer) and log when enabled in builder.go

📍Where to Start

Start with the service registration path in gateway.GatewayServiceBuilder.buildGatewayService in builder.go.

---

Macroscope summarized 27d35bc.

[graphite-app]

How to use the Graphite Merge Queue

Add either label to this PR to merge it via the merge queue:

• Queue - adds this PR to the back of the merge queue
• Hotfix - for urgent hot fixes, skip the queue and merge this PR next

You must have a Graphite account in order to use the merge queue. Sign up using this link.

An organization admin has enabled the Graphite Merge Queue in this repository.

Please do not merge from GitHub as this will restart CI on PRs being processed by the merge queue.

[fbac]

Closes #1006

[mkysel]

that was easy!