feat: ✨ add the backend node for Constant

This requires this PR to be merged:
comfyanonymous/ComfyUI#3329

There is a trick to make it work without that PR but it
feels very hacky, the code for it is kept for reference but unused

nodes/constant.py

@@ -0,0 +1,27 @@
+import json
+
+from ..log import log
+
+
+class MTB_Constant:
+    @classmethod
+    def INPUT_TYPES(cls):
+        return {
+            "required": {"Value": ("*",)},
+        }
+
+    RETURN_TYPES = ("*",)
+    RETURN_NAMES = ("output",)
+    CATEGORY = "mtb/utils"
+    FUNCTION = "execute"
+
+    def execute(
+        self,
+        **kwargs,
+    ):
+        log.debug("Received kwargs")
+        log.debug(json.dumps(kwargs, check_circular=True))
+        return (kwargs.get("Value"),)
+
+
+__nodes__ = [MTB_Constant]