Enable copying tables with generated columns

[postsql]

Skip generated columns as these are not COPY-able

Currently pglogical is unable to replicate tables with generated columns, this pull request fixes this

[nmisch]

Looks reasonable. How feasible is it to add test coverage?

[willibrandon]

This fix looks correct for the COPY path, but there are a couple of additional issues with generated columns beyond initial sync:

1. fill_missing_defaults() in pglogical_apply_heap.c doesn't skip generated columns, so the apply worker can crash when it tries to build_column_default() for a generated column during ongoing replication.

2. ExecComputeStoredGenerated() is never called during INSERT/UPDATE apply, so generated column values aren't being recomputed on the subscriber side.

Might be worth addressing those in the same PR for a complete fix. The ExecComputeStoredGenerated signature also changed between PG 12-13 and PG 14+, so that needs a version guard.