Add DateOnly/TimeOnly support to XmlSerializer and DataContractSerializer

[Copilot]

This PR implements comprehensive support for DateOnly and TimeOnly types in both XmlSerializer and DataContractSerializer, addressing the serialization gap for these .NET 6+ date/time types.

Changes Made

Core Serialization Support

• XmlConvert: Added public ToString(DateOnly), ToDateOnly(string), ToString(TimeOnly), and ToTimeOnly(string) methods
• XmlConverter: Added internal wrapper methods for DataContractSerializer error handling
• XmlSerializer: Added primitive serializers with Write_DateOnly/Read_DateOnly and Write_TimeOnly/Read_TimeOnly methods
• DataContractSerializer: Added DateOnlyDataContract and TimeOnlyDataContract classes with full serialization support

XML Format

• DateOnly serializes using ISO 8601 date format (YYYY-MM-DD)
• TimeOnly serializes using precise time format (HH:mm:ss.fffffff)
• Both formats are compatible with existing XML schema standards and provide full round-trip fidelity

Implementation Details

• Updated type registration systems in both serializers
• Added XML dictionary entries for DataContractSerializer namespace handling
• Extended writer/reader delegators with appropriate conversion methods
• Updated reference assemblies to expose new public APIs

Test Coverage

Added comprehensive tests covering:

• Round-trip serialization for both primitive types and object properties
• Edge cases (min/max values, leap years, precision handling)
• Error handling for invalid input formats
• Default value scenarios
• Both XmlSerializer and DataContractSerializer workflows

Example Usage

// XmlSerializer
var data = new MyClass { Date = new DateOnly(2024, 1, 15), Time = new TimeOnly(14, 30, 45) };
var serializer = new XmlSerializer(typeof(MyClass));
// Produces: <Date>2024-01-15</Date><Time>14:30:45.0000000</Time>

// DataContractSerializer  
[DataContract]
public class MyData 
{
    [DataMember] public DateOnly Date { get; set; }
    [DataMember] public TimeOnly Time { get; set; }
}
var dcs = new DataContractSerializer(typeof(MyData));
// Produces: <DateOnly>2024-01-15</DateOnly><TimeOnly>14:30:45.0000000</TimeOnly>

This implementation maintains backward compatibility and follows existing serialization patterns for other primitive types.

Fixes #56711.

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[teo-tsirpanis]

It's highly doubtful that caching the result of typeof() is helpful; we can remove it either now or in a separate PR.

[jkotas]

Yes, caching typeof like this is a de-optimization in current .NET. It would be best to delete these fields and use typeof directly in all places they are used.

[Lanayx]

It would be magnificent if this could get into .NET 10.

[dotnet-policy-service]

Draft Pull Request was automatically closed for 30 days of inactivity. Please let us know if you'd like to reopen it.