add opportunity to create Hex from rgb

tgbotapi.webapps/src/jsMain/kotlin/dev/inmo/tgbotapi/webapps/Colors.kt

@@ -8,7 +8,9 @@ sealed interface Color {
     value class BackgroundColor(override val value: String) : Color
 
     @Serializable
-    value class Hex(override val value: String) : Color
+    value class Hex(override val value: String) : Color {
+        constructor(r: UByte, g: UByte, b: UByte) : this("#${r.toString(16)}${g.toString(16)}${b.toString(16)}")
+    }
 
     companion object {
         val BackgroundColor = BackgroundColor("bg_color")