This new spec type allows evaluating an arbitrary expression on the
result of a nested spec, for situations where the a value must be
transformed in some way.
This function returns a map describing all of the child block types
declared inside a spec. This can be used for recursive decoding of bodies
using the low-level HCL API, though in most cases callers should just use
Decode which does recursive decoding of an entire nested structure in
a single call.
A BlockLabelSpec can be placed in the nested spec structure of one of the
block specs to require and obtain labels on that block.
This is a more generic methodology than BlockMapSpec since it allows the
result to be a list or set with the labels inside the values, rather than
forcing all the label tuples to be unique and losing the ordering by
collapsing into a map structure.
The main "zcl" package requires a bit more care because of how many
callers it has and because of its two subpackages, so we'll take care
of that one separately.
