Map DateTime between Objo and C# plugin signatures

The C# Plugin SDK's value bridge deliberately maps a small set of signatures (Boolean, Integer, Double, String, MemoryBlock, Picture, arrays, exported enums and exported wrapper classes). System.DateTime — and System.DateTimeOffset — are not on that list, so a plugin author cannot declare an exported member that takes or returns a date/time value directly. The member is rejected at generate time with a diagnostic naming the supported set, never silently, but authors must convert at the boundary themselves (for example passing .NET ticks as Integer, or Unix seconds as Double).

DateTime is a base type that plugin APIs realistically need. Map it so an exported C# member can use DateTime parameters/returns and Objo code sees the native DateTime type on both sides of the call.