Structs can specify the "body" struct tag to capture the entire block
body. This does NOT act as "remain" where leftover fields are
non-erroneous. If you want leftover fields to be allowed, a "remain"
field must ALSO be present.
This is used to capture the full block body that was decoded for the
block. This is useful if you want to ever redecode something differently
(maybe with a different EvalContext) or partially decode something but
redecode the entire value.
Previously we required optional attributes to be specified as pointers so that we could represent the empty vs. absent distinction.
For applications that don't need to make that distinction, representing "optional" as a struct tag is more convenient.
Rather than writing a nil hcl.Expression, as a special case we'll deal
with this within the cty type system, which we assume is what the caller
wants if they are decoding into a hcl.Expression rather than a native Go
type.
This is a super-invasive update since the "zcl" package in particular
is referenced all over.
There are probably still a few zcl references hanging around in comments,
etc but this takes care of most of it.
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.
