(#14137) resource_type serialization naming conventions

This commit attempts to bring the serialized representation
of resource_type objects into better alignment with naming
conventions that we've established in our documentation
and elsewhere. In particular:

• "resource_supertype" => "kind"
• "hostclass" => "class"
• "definition" => "defined_type"
• "arguments" => "parameters"

These changes only affect the serialization (#to_pson/#from_pson).
The internal representation still uses the old terminology.
Ideally we'd just change the internals to conform to the
naming conventions, but that is a larger changeset than we
can reasonably tackle right now.