Added support for UTC and DateTimeOffsets in VEVENT generator

[codebude]

Summary

Inspired by issue #458 and the discussion there, this PR extends the CalendarEvent payload generator with a UTC DateTime functionality.

This PR fixes the following problems:

• CalendarEvent took DateTime objects for start and end of event, but didn't care about DateTime.Kind or any offset

Details

• If plain DateTime objects (with DateTime.Kind.Unspecified) are passed to the payload generator nothing change, to avoid breaking changes
• If DateTime objects with DateTime.Kind.Utc are handed over, the payload generator will respect the UTC and save the dates as "UTC" in the iCal calendar event
• Another constructor overload is added that accepts DateTimeOffset instead of DateTime objects. Those will be converted to DateTime objects with Kind.Utc internally by use of the offset defined in the DateTimeOffset objects.

Test plan

Test cases were added to the PayloadGeneratorTests.cs

Closing issues

Fixes #458

[codebude]

@Shane32 I would be happy if you could review the changes. I'm not sure if I got all of your points from our discussion in #458.

Offtopic: Once more I think timezones are madness... https://www.youtube.com/watch?v=-5wpm-gesOY

[codebude]

Reminder: The wiki should be updated after the merge: https://github.com/codebude/QRCoder/wiki/Advanced-usage---Payload-generators#34-calendar-events-icalvevent

[Shane32]

Looks great! Minor tweak noted. Great video too!

[codebude]

Updated wiki:
https://github.com/codebude/QRCoder/wiki/Advanced-usage---Payload-generators/_compare/974b418e8ed73f14178fb2f7a11460f418ca23f6...dc841758a5891733d79f85858c133b19b76c7f53