SqlServerModelValidator currently blocks UseSequence and UseHilo on non-key properties. We should consider allowing this; we already allow configuring SQL Server IDENTITY on non-key properties, and the user is expressing their intent very clearly, so I'm not sure why we'd block these. For sequences specifically, people can already configure them manually with HasDefaultValueSql, so we're really only blocking our new 7.0 sugar.

(note that Npgsql currently doesn't block these, we should probably align one way or the other)