refactor (controller): Use a grpc server for the grpc-gateway (#1576)

* refactor (controller): Use a grpc server for the grpc-gateway

This starts a grpc server for the grpc-gateway.  The new gateway
server uses an in-memory listener which also supports "dialing", so
the http proxy can connect to the in-memory listener.

This change allows us to use all grpc server features and was in part
motivated by a need for an interceptor of inbound requests.  With that
said, having a "proper" grpc server for the gateway will provide a lot
more flexibility and capabilities moving forward.

The request context will still be verified via middleware in the
http proxy (see wrapHandlerWithCommonFuncs).  This verified context
will be serialized into a protobuf and sent along to the grpc-gateway
via metadata where an interceptor will add the verified request
info to the request's context (see requestCtxInterceptor).  The
interceptor will verify the serialized request info via a shared ticket
with it's companion in-memory http proxy.  FYI: the ticket is simply
random base62 string (see db.NewPrivateId).

This change also necessitated a minor change to services.  We need to
return an empty response instead of nil, nil when there are no errors
and the response is empty.

* refactor (handlers): Rename gRPC Gateway filter

Originally, this was called an OutgoingInterceptor, which is an
unfortunate name since it's not an interceptor.  Now that we're
refactoring the gateway stuff and have proper interceptors, renaming
this should remove ambiguity

* refactor: Rework how 204 with empty msg is handled

Recent changes to grpc gateway has necessitated a change to how
204s are handled by the gateway

* tests (bats): Check status codes for delete operations

internal/errors/code.go

@@ -66,9 +66,11 @@ const (
 	GenKey  Code = 304 // GenKey represents an error occurred during the underlying key generation process
 	GenCert Code = 305 // GenCert represents an error occurred during the underlying certificate generation process
 
-	// General system errors are reserved Codes 400-599 and align with http client and server error codes
-	Forbidden Code = 403 // Forbidden represents the operation is forbidden
-	Internal  Code = 500 // InternalError represents the system encountered an unexpected condition.
+	// General system errors are reserved Codes 400-599 and align with http
+	// client and server error codes
+	Unauthorized Code = 401 // Unauthorized represents the operation is unauthorized
+	Forbidden    Code = 403 // Forbidden represents the operation is forbidden
+	Internal     Code = 500 // InternalError represents the system encountered an unexpected condition.
 
 	// DB errors are reserved Codes from 1000-1999
 	CheckConstraint      Code = 1000 // CheckConstraint represents a check constraint error