Add support of DateOnly/TimeOnly types

[SergeiPavlov]

Experimental implementation for MSSQL only

Enabled by conditional compilation for .NET6

It can be disabled by setting env var DO_DATEONLY=false or by property in User.Directory.Build.props

[alex-kulakov]

This is great PR! I appreciate it. I'll merge it to a proxy branch where I add support for these times to other providers.

[alex-kulakov]

But instead of DO_DATEONLY I would use the built-in NET6 compile time variable, so I will remove this env var mentions from the code, are you OK with it?

[SergeiPavlov]

But instead of DO_DATEONLY I would use the built-in NET6 compile time variable, so I will remove this env var mentions from the code, are you OK with it?

It would be Ok for us but not for other DO users.
DateOnly type is supported for SqlServer only.

But if you can implements other providers, then OK

With other DB engine the tests will fail under .NET6 without DO_DATEONLY conditional compilation

[alex-kulakov]

SQL's date and time are supported by the majority of RDBMSs. Client libraries can be a problem, but we can do the same as we did for DateTimeOffset - make them unsupported. But I think we will have no problems especially if there is a way to read SQL's date as DateTime value and time as something else and then convert them to DateOnly and TimeOnly respectively.