Add support for a custom prefix for Union type fields in Avro when enableStableIdentifiersForUnionType is enabled

enableStableIdentifiersForUnionType allows to enable stable identifiers in Avro. We need to add another config to allow configuring the custom prefix used in stable identifiers. Currently the value is member_, e.g. member_int, member_string, but we should be able to change to any value or even leave empty.