feat: potentially add xmin replication?

Airbyte has this as of July 10, see here. Haven't spent time scoping this out but doesnt work on current version of this plugin. I suspect this'd be easy to add, I assume the catalog is excluding hidden/system columns and it could just include them(?)

Specifically, I tried to just set xmin as the replication column, and it looks like it errors saying hey that column isnt in the catalog.

2023-08-25T17:06:34.041196Z [info     ] time=2023-08-25 17:06:31 name=tap_postgres level=INFO message=Stream public-mytable is using incremental replication with replication key xmin cmd_type=elb consumer=False name=pipeline-0 producer=True stdio=stderr string_id=pipeline-0
2023-08-25T17:06:34.041319Z [info     ] time=2023-08-25 17:06:31 name=tap_postgres level=CRITICAL message='NoneType' object has no attribute 'get' cmd_type=elb consumer=False name=pipeline-0 producer=True stdio=stderr string_id=pipeline-0
...
2023-08-25T17:06:34.043415Z [info     ]     replication_key_sql_datatype = md_map.get(('properties', replication_key)).get('sql-datatype') cmd_type=elb consumer=False name=pipeline-0 producer=True stdio=stderr string_id=pipeline-0
2023-08-25T17:06:34.043529Z [info     ] AttributeError: 'NoneType' object has no attribute 'get' cmd_type=elb consumer=False name=pipeline-0 producer=True stdio=stderr string_id=pipeline-0

Start by tracing how the catalog populates metadata in md_map and how replication_key_sql_datatype looks up xmin, using the reported NoneType error as the entry point. Compare the current behavior with the linked Airbyte implementation, then verify that xmin can be selected as a replication column without the catalog lookup failing.