Add a DataType.identity() flag to specify that a data type is meant to be used as a SERIAL / IDENTITY type

[lukaseder]

This is a prerequisite for #5062.

Currently, the only place where a Table's IDENTITY flag is stored is the Table itself, and the Identity reference contains a reference to the Field.

This doesn't reflect well what we do in DDL statements, when we specify for a column to be an Identity column. We do that as a type modifier, similar to NOT NULL (which is a type constraint directly placed on the type, at least syntactically).

In order to be able to implement #5062, it would be great if we could say something like:

create().createTable("t")
        .column(field(name("t", "i"), Integer.class), SQLDataType.INTEGER.identity(true))
        .column("n", DECIMAL(3, 1).nullable(true))
        .column(field(name("s"), VARCHAR(5).nullable(false)))
        .execute();

Some databases support additional sequence flags, including START WITH or STEP. We'll always generate the default values for those databases, as most databases don't support these flags.

[lukaseder]

Setting the identity flag should unset the defaultValue() and the nullable() flag and vice versa