Add support for Dataverse custom PowerFx functions

[mkholt]

Add support for in the PowerFx parser for:

• IsUTCToday
• ISOWeekNum
• UTCNow
• UTCToday

Also added tests for all other functions supported by Dataverse, as mentioned in the documentation: https://learn.microsoft.com/en-us/power-platform/power-fx/formula-reference-formula-columns

[Copilot]

Pull Request Overview

This PR adds support for custom PowerFx functions specific to Dataverse, implementing UTCToday, UTCNow, IsUTCToday, and ISOWeekNum functions. It also includes comprehensive test coverage for these new functions along with existing supported PowerFx functions.

Key changes:

• Implemented four custom PowerFx functions for date/time operations with UTC timezone support
• Added TimeSpan offset parameter to the FormulaFieldEvaluator for testing flexibility
• Refactored test infrastructure to use a service factory pattern for better abstraction

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/XrmMockup365Test/UnitTestBase.cs	Refactored to extract service wrapper and add organization service factory pattern
tests/XrmMockup365Test/TestFormulaFields.cs	Added comprehensive test coverage for PowerFx functions including new Dataverse-specific functions
src/XrmMockup365/FormulaFieldEvaluator.cs	Enhanced to support custom functions and time offset for testing
src/XrmMockup365/CustomFunction/UTCTodayFunction.cs	Implements UTCToday custom function
src/XrmMockup365/CustomFunction/UTCNowFunction.cs	Implements UTCNow custom function
src/XrmMockup365/CustomFunction/IsUTCTodayFunction.cs	Implements IsUTCToday custom function
src/XrmMockup365/CustomFunction/ISOWeekNumFunction.cs	Implements ISOWeekNum custom function with .NET Framework compatibility
src/XrmMockup365/Core.cs	Updated to pass time offset to formula evaluator

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