The avro_schema_prefix flag avoids collisions in the subject, but records or
lower-level fields could still collide. This PR sets the "namespace" property
on avro schemas to be equal to the prefix when provided.
Note that this slightly changes encoding: if the prefix "super" is provided,
"super." will be prepended to the keys of the subfields of an avro record,
such as the "after" part of an emitted value.

Release justification: low-risk update to behavior for a new flag
Release note (enterprise change): avro_schema_prefix flag sets Schema.namespace