Add support for formula fields

[StepanHoudek]

This pull request adds support for marking entity attributes as formula fields in the Dataverse Power Platform entity attribute template. The changes introduce a new IsFormulaField parameter that, when enabled, sets the appropriate metadata and includes a formula definition file for the attribute.

Enhancements for formula field support:

• Added a new boolean parameter IsFormulaField to template.json, allowing users to specify if an attribute is a formula field. This parameter is only enabled for supported attribute types and sets relevant metadata when true.
• Updated file exclusion rules in template.json so that the formula definition YAML file is only included when IsFormulaField is true.

Template rendering logic updates:

• Modified attribute.xml to set <SourceType> to 3 for formula fields and to 0 otherwise, based on the IsFormulaField parameter.
• Added conditional inclusion of <FormulaDefinitionFileName> in attribute.xml when the attribute is a formula field.

[StepanHoudek]

Formula fields are only supported for these data types:

Text
Decimal Number
Whole Number
Float
Boolean Choice (Yes/No)
Choice (formerly option sets)
Datetime

Next step is to stop the formula attribute creation if the data type doesn't match. Current CLI only supports Post Actions, I suggest implementing Pre Actions for this check.

[TomProkop]

My concerns:

1. Does it work when solution-root-path is undefined (solution files unpacked at the csproj root level)
2. The template.json symbol descriptions are the only hint that a coding agent gets. It should be a bit more descriptive so that agents understand what goes into the formula definition file
3. RE your previous comment: Shouldn't the isEnabled handle that?

Other than that LGTM.

[StepanHoudek]

1. Works in this scenario

2. Modified

3. It wasn't being evaluated for the .yaml file. Added a computed field which gets always evaluated.