Add support for DateOnly/TimeOnly into xml serializer

[rbev]

Fix for #7126

[tmasternak]

I'm leaning towards the approach indicated by Daniel - explicit type handling

[danielmarbach]

I dug a little deeper because I was wondering how DateTime and DateTimeOffset are handled internally. I found out that XElement has native support for that and started wonder why we even have to do anything. This made me find this gem dotnet/runtime#119835

[danielmarbach]

Raised dotnet/runtime#126879.

[danielmarbach]

Looking at the .NET runtime PR, the wire formats are:

• DateOnly → yyyy-MM-dd
• TimeOnly → HH:mm:ss[.FFFFFFF] (trimmed trailing zeros, not full roundtrip format)

Our current "O" format for TimeOnly produces 14:30:25.1230000 with trailing zeros, while the runtime uses a minimal form like 14:30:25.123.

Should we use yyyy-MM-dd and HH:mm:ss.fffffff (trimmed) instead of "O" to match the .NET runtime behavior? Or is our current "O" format acceptable since it's also a valid roundtrip format?

[rbev]

I dug a little deeper because I was wondering how DateTime and DateTimeOffset are handled internally. I found out that XElement has native support for that and started wonder why we even have to do anything. This made me find this gem dotnet/runtime#119835

Yeah i saw that one too and was surprised that XElement was still using the plain .ToString() when they had added support to the other XML serialisers.

[rbev]

Should we use yyyy-MM-dd and HH:mm:ss.fffffff (trimmed) instead of "O" to match the .NET runtime behavior? Or is our current "O" format acceptable since it's also a valid roundtrip format?

I think "O" would be acceptable - It's not going to break anyone, but I think the trimmed version would be slightly better as it will generally drop 3 or more bytes from each message.