Add parameterless overload of HasDefaultValueSql

[ajcvickers]

HasDefaultValueSql does two things:

• It tells Migrations what to set on the column.
• It tells the update pipeline that there is default for the column so that it can do the correct thing on inserts.

If you are managing the database outside of EF, then the first point doesn't apply, but you still need to pass in some string, even though this is then ignored by the update pipeline. This has always seemed a bit clunky. Instead we could allow a parameterless call to be made:

modelBuilder.Entity<Blog>()
        .Property(b => b.Created)
        .ValueGeneratedOnAdd()
        .HasDefaultValueSql();

I think this looks pretty nice in the fluent API and is less clunkly than passing some random string.

If this is used with Migrations/EnsureCreated, it could throw, or maybe it could just default to using null for the default value.

/cc #4189

[ajcvickers]

Design meeting notes:

• We decided to do this as described, except use the HasDefaultValue() method rather than the HasDefulatValueSql() because the latter says "there is a default value of some form" without it necessarily being SQL.