Enable Specifying Role Assignment Principal Type

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### Description

I would like have the ability to control the `principal_type` value set in all of the module's role assignments, whether implicitly or explicitly (preferably).

**Example:**
https://github.com/Azure/terraform-azurerm-caf-enterprise-scale/blob/0783a8a720ed4d950c10006bfa845eb59155f334/resources.role_assignments.tf#L1

#### Is your feature request related to a problem?

Yes, it is a security best practice to utilize Groups instead of assigning individual Users role assignments. Using this module to define and deploy a policy that enforces this concept by blocking creation of role assignments targeting individual users requires that any future role assignments explicitly set the `principal_type`. However, this module does not expose any mechanism to pass the `principal_type` or do any internal checks against Entra to determine the appropriate value to set, effectively breaking the module's capability to manage role assignments.

#### Describe the solution you'd like

At least one of two mechanisms should be provided:

- Internally do a lookup to determine the appropriate value and automatically set the `principal_type` for every role assignment (less consumer impact, but likely more challenging)
- Provide an optional attribute in the archetype JSON. If not explicitly provided, default to the existing behavior of not supplying a value to the underlying resource.
```
"access_control_v2" = {
"Contributor Users" = {
"principal_type" = "User"
"members" = ["principal ids"]
}
"Contributor Groups" = {
"principal_type" = "Group"
"members" = ["principal ids"]
}
"Contributor SPNs" = {
"principal_type" = "ServicePrincipal"
"members" = ["principal ids"]
}
}
```

#### Additional context

- This feature request is somewhat related to Azure/Azure-Landing-Zones#514, and it could potentially share a common implementation. However, it could be handled separately if implementation is complex, or there is a desire to keep the functionality independent.

- While this request is not currently a blocking issue that would classify it as a bug, it is reasonable to assume this module will require addressing this functionality eventually. The Azure CLI already makes note that this is likely a future breaking change in the REST API with the following message:
`RBAC service might reject creating role assignment without --assignee-principal-type in the future. Better to specify --assignee-principal-type manually.`

Review the linked resources.role_assignments.tf example and the archetype JSON structure described in the issue, then trace how role assignments are generated in the module. Confirm how an optional principal_type should flow to role assignments while preserving existing behavior when it is absent; done means users can specify User, Group, or ServicePrincipal values for the assignments they define.

Written by the indexing model from the issue text.