[Feature Request] `Choice` spec

[kurtamohler]

Motivation

Add a Choice spec that chooses between a stack of specs or non-tensor data.

Initially proposed here: #2698 (comment)

Solution

When generating a sample, Choice spec should choose between the specs in the stack and then generate a sample from that spec.

Alternatives

Additional context

Checklist

• I have checked that there is no similar issue in the repo (required)

[kurtamohler]

Closed by #2713