Allow choicesUrl to include parts of the current JSON object

[aeberhart]

In forms, the choices of one input field often depend on another field. For instance, choosing a country (e.g. US) would restrict the state choices to AL, AR, AK, ....

We could achieve this in two ways / syntaxes:

1. JavaScript template literal (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Template_literals):
   choicesUrl = /get/states/$ {country}. This is more readable but raises the question what kind of expressions are allowed. For instance, Can we use a more complex expression like $ {address[1].state}?

2. Allowing choicesUrl to be computed using a JSONata expression, similar to this "compute" example:
   https://dashjoin.github.io/#/example/compute. Less readable, since the expression requires JSONata string concat and escaped quotes. We have the full flexibility though.