Fix pickle round-trip of specs propagating variants (#47351)

This changes `Spec` serialization to include information about propagation for abstract specs.
This was previously not included in the JSON representation for abstract specs, and couldn't be
stored.

Now, there is a separate `propagate` dictionary alongside the `parameters` dictionary. This isn't
beautiful, but when we bump the spec version for Spack `v0.24`, we can clean up this and other
aspects of the schema.