Provide a Remap operation for JSON diagrams

[mxgrey]

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.