schema_extension allows making invisible changes to system_schema
that evade upgrade rollback tests. They appear in system_schema
as an encoded blob which reduces serviceability, as they cannot
be read.
Deprecate it and point users to adding explicit columns in scylla_tables.
We could probably make use of the data structure, after we teach it
to encode its payload into proper named and typed columns instead of
using IDL.
Closesscylladb/scylladb#23151
a62ab824e6