Getting subscription and service limits via Powershell

## Description of the new feature
We have a list of our subscription and service limits for various resources at: https://docs.microsoft.com/en-us/azure/azure-resource-manager/management/azure-subscription-service-limits
It would be great to be able to query these limits directly to compare them against current usage at a subscription or resource level.

## Proposed implementation details (optional)
Right now finding your current usage vs limits via Powershell is only supported for specific resources: VMs (Get-AzVMUsage), Storage(Get-AzStorageUsage) & Networking(Get-AzNetworkUsage). It would be great to expand on this to include other services as well. One example use case for these commands is in environments with many deployments and complexities to find if subscription/limits may be reached soon to act ahead of time in preparation for tasks such as large migrations.