XmlSerializer dont skip date time offset (#55101)

XmlSerializer dont skip date time offset (#55101)

* Add DateTimeOffset as a primitive for XmlSerializer.

* Fix DefaultValueAttribute handling for DTO.

* Add tests for DateTimeAttribute and XmlSerializer.

* Missed a 'KnownType' spot.

* Update SGen tests to use current dotnet.exe from build.

* Unbreak runtimeconfig creation for testing.

* Debugging failure to resolve CoreCLR path.

* Test fixup.

* More test cleanup.

* Temporarily disable generator tests.

* Use InitObj instead of constructors for DateTimeOffset and TimeSpan.