Implement IDENTITY column

The IDENTITY column feature as per SQL standard as well as at least Oracle 12 and up and DB2:

CREATE TABLE tab1(c1 INTEGER GENERATED ALWAYS AS IDENTITY,

   c2 INTEGER GENERATED BY DEFAULT AS IDENTITY);

Oracle supports similar options as a SEQUENCE to an IDENTITY column. An Identity column need not be a primary key, but it can be. With GENERATED ALWAYS is used, the user cannot specify a value, only the generated value is allowed.

Oracle-21 syntax for IDENTITY columns:

identity_clause::=  GENERATED [ ALWAYS | BY DEFAULT [ ON NULL ] ] AS IDENTITY [ ( identity_options ) ]

identity_options::=   { START WITH ( integer | LIMIT VALUE )

                    | INCREMENT BY integer

                    | ( MAXVALUE integer | NOMAXVALUE )

                    | ( MINVALUE integer | NOMINVALUE )

                    | ( CYCLE | NOCYCLE )

                    | ( CACHE integer | NOCACHE )

                    | ( ORDER | NOORDER ) }...

Note from monty:
IDENTITY should parse and ignore all the options. The behavior should be controlled by auto_increment_increment and auto_increment_offset.

Note from serg:
IDENTITY should be implemented as Field::default_value. GENERATED ALWAYS AS IDENTITY is also a default_value (since vcols can only be deterministic), but not user-assignable.