avro-arrow conversion support union type with multiple non-null types

This will greatly simplify in creating arrow record batch for delete log block, as noted in `avro_schema_for_delete_record()`

Start by locating avro_schema_for_delete_record() and the Avro-to-Arrow conversion code it uses. Trace how unions are handled, then verify that unions containing multiple non-null types can produce an Arrow record batch for the delete log block; the issue names no tests or specific files to run.

Written by the indexing model from the issue text.