Get-AzSubscription always gets all subscriptions even with SubscriptionId parameter and then filters

## Description of the new feature
If the parameter "SubscriptionId" is specified, the commandlet should use the REST call dedicated to that instead of retrieving all subscriptions and filtering internally. If a tenant has a lot of subscriptions, and the user calling even has access to several tenants, the commandlet can time out frequently. You can verify the slow behavior with the "debug" parameter

## Proposed implementation details (optional)
See https://docs.microsoft.com/en-us/rest/api/resources/subscriptions/get