Manage Subscriptions

[DSpirit]

As a developer, I would like to be able to manage subscriptions in the VSCode extension, so that I don't need to use the master key obtained from the "right-click copy subscription-key".

1. Show subscriptions
2. List subscriptions / List by product
3. Create subscriptions (also "admin" subscriptions for tracing reasons)
4. Delete subscriptions

[RupengLiu]

add subscription tree item

[miaojiang]

In progress