fix: S3 gateway doesn't support full passthrough for encryption (#10484)

The entire encryption layer is dependent on the fact that
KMS should be configured for S3 encryption to work properly
and we only support passing the headers as is to the backend
for encryption only if KMS is configured.

Make sure that this predictability is maintained, currently
the code was allowing encryption to go through and fail
at later to indicate that KMS was not configured. We should
simply reply "NotImplemented" if KMS is not configured, this
allows clients to simply proceed with their tests.

cmd/routers.go

@@ -108,9 +108,8 @@ func configureServerHandler(endpointZones EndpointZones) (http.Handler, error) {
 		}
 	}
 
-	// Add API router, additionally all server mode support encryption
-	// but don't allow SSE-KMS.
-	registerAPIRouter(router, true, false)
+	// Add API router
+	registerAPIRouter(router)
 
 	router.Use(registerMiddlewares)