Wrap serialized gradient stops in a new GradientRamp struct and unify FillChoice (#4400)

* Introduce the GradientRamp exchange struct as the serialized TaggedValue::Gradient payload

* Unify FillChoice and FillChoiceUI into one enum generic over color format, carrying GradientRamp stops

* Rename the TaggedValue::Gradient variant to GradientRamp to match its payload

* Move the Color variant into the tagged_value macro list since its stored and wire forms match

node-graph/graph-craft/src/proto.rs

@@ -1320,7 +1320,7 @@ mod test {
 		// If this assert fails: These NodeIds seem to be changing when you modify TaggedValue, just update them.
 		assert_eq!(
 			ids,
-			vec![NodeId(12331852515109999872), NodeId(5084548161767585362), NodeId(14635346976242256925), NodeId(16015195863711239715)]
+			vec![NodeId(8464972237805743576), NodeId(3528778906331798968), NodeId(1126597937993520391), NodeId(17582929706900579130)]
 		);
 	}