ext/typeexpr: Added support for optional object attributes to TypeConstraint. Attributes can be wrapped in the special optional(…) modifier, allowing the attribute to be omitted while still meeting the type constraint. For more information, cty's documentation on conversion between object types. (#549)
ext/typeexpr: New function: TypeConstraintWithDefaults. In this mode, the optional(…) modifier accepts a second argument which can be used as the default value for omitted object attributes. The function returns both a cty.Type and associated Defaults, the latter of which has an Apply method to apply defaults to a given value. (#549)
