New-UnifiedEnvironment in environment group #133

[FH-Inway]

This pull request introduces the ability to specify an environment group when creating a new Unified Environment, making the environment a managed environment in the specified environment group. It does this by adding a new -EnvironmentGroup parameter to the New-UnifiedEnvironment function, supporting both group IDs and display names, and by implementing a new Get-PpacEnvironmentGroup cmdlet to retrieve available environment groups. The changes also update the documentation and ensure the environment group is passed through to the shell environment creation logic.

Implements #133

[FH-Inway]

Update to resolve validation errors after #131 is merged, it contains scripts to generate the required changes.