Add DateTime Extensions methods

[WaldemarKaib]

There are several extensions methods for the DateTime struct we use in our project. These might be handy for others as well.
We asked ourselves which method we would use in a second project and before copying them or creating a nuget package, integrating these methods into Arc4u seems a good way.

Extensions methods:

• GetFirstHourOfDay
• GetLastHourOfDay
• GetEndOfDay
• EachHour
• GetTotalHoursOfDay
• TruncateMilliseconds

[rdarko]

We arent able to see the value of having such methods to support a handful of projects. We also need to take DateTimeOffset into account, because we need to provide the same method here (for simetry) and we also need to investigate how it combines to DateOnly and TimeOnly types introduced in .net 6. Although a good suggestion, for the time being, such dateTime support isnt on the roadmap of arc4u.