config: allow a HCL2 body to piggy-back on a RawConfig
config: allow a HCL2 body to piggy-back on a RawConfig

At this time we're not ready to refactor the various uses of RawConfig
in Terraform core, so we'll smuggle a HCL2 body within a degenerate
RawConfig object that we can then recognize and unpack once this object
is returned to us in an interpolation call.

hashicorp/terraform-plugin-sdk,hashicorp/terraform-plugin-sdk