feat: add Proxy channel type and relay mode (#1678)

Add the Proxy channel type and relay mode to support proxying requests to custom upstream services.

relay/relaymode/helper.go

@@ -24,6 +24,8 @@ func GetByPath(path string) int {
 		relayMode = AudioTranscription
 	} else if strings.HasPrefix(path, "/v1/audio/translations") {
 		relayMode = AudioTranslation
+	} else if strings.HasPrefix(path, "/v1/oneapi/proxy") {
+		relayMode = Proxy
 	}
 	return relayMode
 }