Add parser support for VERTICA IDENTITY column type

[lukaseder]

In Vertica, an IDENTITY column has an implied data type:

create table "t" ("i" identity[*](1, 1) not null, "s" varchar(5) not null, primary key ("i"))

But our parser cannot process this:

create table "t" ("i" identity[*](1, 1) not null, "s" varchar(5) not null, primary key ("i"))

See also: https://www.vertica.com/docs/9.2.x/HTML/Content/Authoring/AdministratorsGuide/Sequences/AutoIncrementIdentitySequences.htm