MobileFormatter should serialize DateTimeOffset

[rockfordlhotka]

See #2522 for discussion.

The DateTimeOffset type is not normally serializable. MobileFormatter should be enhanced to serialize DateTimeOffset.

Also, the analyzer that checks for serializable types should be enhanced to understand that this type can now be serialized.

[swegele]

When whoever gets to coding this...they should probably .ToString the DateTimeOffset in the "O" round-trip format as talked about here, to preserve all original information contained therein.

https://docs.microsoft.com/en-us/dotnet/standard/base-types/standard-date-and-time-format-strings#the-round-trip-o-o-format-specifier

[rockfordlhotka]

Maybe can be resolved once #2531 is complete?