[Enhancement] more options for `FieldRename`

Thank you for providing the `FieldRename` enum. It quickly helps for common conventions.

It's great, but some backends adopt some weird naming conventions. I recently had to interface my client towards a server that returns only `lowercase` json fields, e.g.
```json
{
"somevaluewithnocase": 9
}
```

The fact that this is even a thing is very much debatable, thus making this request kind-of low-priority, but I was wondering if we could add more options for `FieldRename`, like for this one, above.

One good and complete list might be the following from mason. As you can see, there, we're pretty much allowed to do anything (obviously, it's a completely different context, and indeed there's some overlap, but you get the general idea).
https://docs.brickhub.dev/brick-syntax#built-in-lambdas