Add converters to DatePicker and TimePicker for DateOnly and TimeOnly

[symbiogenesis]

I realize that full DateOnly and TimeOnly support is not implemented yet. This is a start in that direction.

Fixes #20438

As mentioned in the comments there, fixing that bug is merely a subset of #1100 which can be fixed without the need for a full implementation.

[dotnet-policy-service]

Hey there @symbiogenesis! Thank you so much for your PR! Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[jsuarezruiz]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 3 pipeline(s).

[jsuarezruiz]

There are some compilation errors:

D:\a\_work\1\s\src\Controls\src\Core\TimePicker\TimePicker.cs(71,47): error CS0246: The type or namespace name 'TimeOnlyConverter' could not be found (are you missing a using directive or an assembly reference?) [D:\a\_work\1\s\src\Controls\src\Core\Controls.Core.csproj::TargetFramework=net8.0-windows10.0.19041.0]