CDC update on tables with column type as "generated always as identity" fails

[arajkumar]

Updating a row value in the following schema fails with error

create table metrics( "SerialNumber" integer generated always as identity, name text);

ERROR:  column "SerialNumber" can only be updated to DEFAULT
DETAIL:  Column "SerialNumber" is an identity column defined as GENERATED ALWAYS.

Currently, we handle only:

• Generated columns with is_generated = 'ALWAYS' for INSERT and UPDATE (worked on #664 )
• IDENTITY columns for INSERT using "overriding system value"