[Feature]: get-azapimanagementpolicy support for Policy Fragments

### Description of the new feature

As of Powershell Module Az.ApiManagment v12.4.0 the [get-azapimanagementpolicy](https://learn.microsoft.com/en-us/powershell/module/az.apimanagement/get-azapimanagementpolicy?view=azps-12.4.0) command doesn't support retrieval of Policy Fragments. Please add parameters similar to the existing (-ProductId, -ApiId etc) ones that will enable retrieval of Policy Fragments.

### Proposed implementation details (optional)

Add a new parameter -FragmentId to the relevant commands to enable retrieval of a policy fragment presumably via the [Policy Fragment Resource Management API](https://learn.microsoft.com/en-us/rest/api/apimanagement/policy-fragment/get?view=rest-apimanagement-2024-05-01&tabs=HTTP)

Start with the Get-AzApiManagementPolicy command in the Az.ApiManagement module and compare its existing ProductId and ApiId parameters with the Policy Fragment Resource Management API documentation linked in the issue. Add FragmentId support for retrieving policy fragments, then verify that the command returns the requested fragment.

Written by the indexing model from the issue text.