[Feature]: Add Subscriptions to APIs and Products

[simonkurtz-MSFT]

Describe the feature

Add API Management subscriptions that are tied correctly to APIs and products.

Improvement to Project

This improves security and can ensure that only certain products can be used as is the case with the HR product for the auth samples.

Are you able to collaborate and/or submit a pull request?

Yes

[simonkurtz-MSFT]

Product subscriptions should be prefixed with product- followed by the product name.
API subscriptions should be prefixed with api- followed by the API name.