Provide a Remap operation for JSON diagrams

Currently the CEL transform operation is a highly flexible way to remap one data structure into another, but it has a critical drawback: It only works for serializable and deserializable messages.

Some messages may include fields that are not serializable or deserializable, but that still need to undergo some kind of remapping, e.g. change of field names, change between a tuple and a struct. This will be especially important for tuple-based operations like inject.

We could provide a remapping utility, either as its own JSON diagram operation or as an alternative to CEL for the existing Transform operation.

We don't need to do anything for the native Rust API since it's already easy enough for a user to write a map_block node.

Start with the existing CEL transform operation and the JSON diagram operation described in the issue. Compare whether remapping should be a separate operation or an alternative to Transform, using the existing Rust map_block node as the native API reference; done means the scope and implementation direction for non-serializable messages are agreed.