Our support for the Guid type is currently spotty. Some providers support it natively (e.g. PostgreSQL, SQL Server); for those that don't, some support it by storing it as a string, but others don't. We should probably allow Guids everywhere, as they're mappable to string without loss of information etc.

Note: pay attention to uppercase vs. lowercase representation of Guids as strings - lowercase is probably the way to go (though EF generates uppercase by default, see dotnet/efcore#19651).