Add optional COLUMN to ALTER TABLE ... ADD and DROP

[mrotteveel]

The SQL:2023 syntax for the ALTER TABLE actions <add column definition> and <drop column definition> are defined as:

<add column definition> ::=
  ADD [ COLUMN ] <column definition>

<drop column definition> ::=
  DROP [ COLUMN ] <column name> <drop behavior>

Firebird already allows an optional COLUMN in <alter column definition>.

This PR adds support for optional COLUMN for ADD and DROP as well.