[Feature]: New-AzRoleAssignment cmdlet should support SupportsShouldProcess

### Description of the new feature

New-AzRoleAssignment cmdlet should support `-Confirm` and `-WhatIf` to maintain parity with Set-AzRoleAssignment and `Remove-AzRoleAssignment`.

I was writing a script and wanted to use -Confirm for or RBAC write but noticed that `New-AzRoleAssignment` does not support it even `Set-AzRoleAssignment` does. This makes scripting really weird.

* Looks like `SetAzureRoleAssignmentCommand` was added with confirmation support but it was never added into `New-AzRoleAssignment`
* Not sure about history of `Remove-AzRoleAssignment`

### Proposed implementation details (optional)

_No response_

Inspect the New-AzRoleAssignment cmdlet implementation and compare it with SetAzureRoleAssignmentCommand and Remove-AzRoleAssignment, especially their confirmation support. Verify how SupportsShouldProcess, -Confirm, and -WhatIf are exposed and exercised, then run the relevant role-assignment tests if available. Done means New-AzRoleAssignment provides behavior consistent with the other role-assignment cmdlets.

Written by the indexing model from the issue text.