source-{postgres,sqlserver}: Improve geospatial type handling

[willdonnelly]

In the past we've mostly punted on geospatial type support, but in #3808 I went ahead and implemented support in source-mysql and source-mysql-batch for capturing MySQL's geospatial datatypes as WKT strings and it really wasn't that bad. We should look into doing the same for Postgres and SQL Server.