Implement conditionals

[japsu]

Use cases:

• If the condition is met, return this value, otherwise return that value
• If the condition is met, define this key in a dict, otherwise leave it out
• If the condition is met, include this element in a list, otherwise leave it out

Simple conditions (Python truthiness of a value) are easy to implement. How about complex predicates (==, <, <=, >, >= etc.)?

Switching to a JSONPath implementation that supports ? is one option. Then we could just use !Lookup in the condition.

We do not want to execute Python to resolve the condition.

[akx]

I think we should support most of the operators exposed by the operator module, because it's easy to do.

In addition, AND and OR need to be available somehow. Trees thereof maybe not, so maybe allOf and anyOf for conditions.

[japsu]

Original CoA met, closing. Create new issues of missing functionality if any.