fix: preserve HTTPRoute upstream scheme from backend ref

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: AlinsRan

internal/adc/translator/httproute.go

@@ -572,7 +572,9 @@ func (t *Translator) TranslateHTTPRoute(tctx *provider.TranslateContext, httpRou
 
 			t.AttachBackendTrafficPolicyToUpstream(backend.BackendRef, tctx.BackendTrafficPolicies, upstream)
 			upstream.Nodes = upNodes
-			upstream.Scheme = appProtocolToUpstreamScheme(protocol)
+			if upstream.Scheme == "" {
+				upstream.Scheme = appProtocolToUpstreamScheme(protocol)
+			}
 			var (
 				kind string
 				port int32