Ignore existing resources within a subscription

Add flag to cancel subscription request to ignore currently existing (deployed) resources in the subscription to be cancelled.

Originally https://github.com/Azure/azure-rest-api-specs/issues/9042

see the discussion at https://github.com/Azure/azure-sdk-for-python/issues/10814#issuecomment-760670103

For me it's far safer for us to 'soft-delete' a subscription, by cancelling it than deleting the resources.

It can be very hard to tell if a subscription is still in use when you have so many, but when you cancel it you find out within the 3 months.

Deleting the resources is far riskier.

Of course this should be hidden behind a flag and only for people who are aware of the implications.

Start with the linked Azure REST API issue 9042 and the discussion in azure-sdk-for-python issue 10814, then locate the subscription cancellation request specification in this repository. Confirm the opt-in flag's intended behavior for subscriptions with deployed resources and update the relevant API definition so cancellation can ignore those resources without deleting them.

Written by the indexing model from the issue text.