Add option to mark non-primary columns as database generated.

[TwentyFourMinutes]

Any column should be able to be marked as database generated.

API Proposal

As the configuration possibilities for a single column stack up by now, we should consider adding a new fluent API just for columns.
This could look something like this:

entityBuilder.Property(x => x.Column).WithName("column");

With the option to mark it as database generated:

entityBuilder.Property(x => x.Column).HasDefault();

// Or

entityBuilder.Property(x => x.Column).IsGenerated();

[TwentyFourMinutes]

Related to #13.