The optional modifier previously accepted a single argument: the
attribute type. This commit adds an optional second argument, which
specifies a default value for the attribute.
To record the default values for a variable's type, we use a separate
parallel structure of `typeexpr.Defaults`, rather than extending
`cty.Type` to include a `cty.Value` of defaults (which may in turn
include a `cty.Type` with defaults, and so on, and so forth).
The new `typeexpr.TypeConstraintWithDefaults` returns a type constraint
and defaults value. Defaults will be `nil` unless there are default
values specified somewhere in the variable's type.
This builds on an experimental feature in the underlying cty library which
allows marking specific attribtues of an object type constraint as
optional, which in turn modifies how the cty conversion package handles
missing attributes in a source value: it will silently substitute a null
value of the appropriate type rather than returning an error.
In order to implement the experiment this commit temporarily forks the
HCL typeexpr extension package into a local internal/typeexpr package,
where I've extended the type constraint syntax to allow annotating object
type attributes as being optional using the HCL function call syntax.
If the experiment is successful -- both at the Terraform layer and in
the underlying cty library -- we'll likely send these modifications to
upstream HCL so that other HCL-based languages can potentially benefit
from this new capability.
Because it's experimental, the optional attribute modifier is allowed only
with an explicit opt-in to the module_variable_optional_attrs experiment.
