Remove Dependency on Azure Account extension

[peterbom]

Is your feature request related to a problem? Please describe.
There are some problems with depending on the Azure Account extension:

1. It adds extra 'weight' to our extension, by forcing users to install something they may not want or use.
2. It is now deprecated.
3. Upcoming work that requires Azure credentials will make our extension more deeply coupled to the Azure Account extension, making it harder to make these changes later.
4. There is a long-standing bug with it that prevents us retrieving credentials needed for other Azure SDKs in the recommended manner (using a VisualStudioCodeCredential)
5. It uses a separate sign-in and credential handling experience from the VS Code Authentication API. This means it doesn't pick up the user's session(s) visible in the Account menu:
   

Mention what platform you want to support the new feature
All platforms

Describe the solution you'd like

• Users not to be forced to install the Azure Account extension
• To be able to use the VS Code API for getting authentication sessions
• For the Account view to match the Cloud Explorer tree view

Describe alternatives you've considered
None, although the approach is discussed in the associated PR.

Additional context

[peterbom]

This will be addressed by #551. Some notes about the changes in appearance and behaviour:

1. After first upgrading to the version of the extension which has these changes, users will no longer be signed in on the Cloud Explorer tree view, which will look like this:
   
2. After selecting the option to sign in (either from the tree view itself or using the "AKS: Sign in to Azure..." command), users will be presented with a confirmation dialog:
   
3. Due to a limitation of the VS Code authentication API there is no way for our extension to cancel an authentication request after launching a browser tab. That means if users close that tab without completing the sign-in, Cloud Explorer will get stuck in "waiting" state until the VS Code window is reloaded:
   
4. To switch between tenants users will need to run the "AKS: Sign in to Azure..." command and select "Sign in to another account":
   
5. If users had been filtering the subscriptions in the Cloud Explorer tree view using the "Select subscriptions" functionality of the Azure Account extension, those filters will continue working. These are stored by the Azure Account extension in the azure.ResourceFilter setting:
   
6. The selected subscriptions can be changed using the "AKS: Select subscriptions" command:
   
7. If the selected subscriptions are changed, they will be stored in user settings under the aks.selectedSubscriptions section:
   
8. The VS Code "Accounts" menu will now contain the account used to sign in to the Cloud Explorer tree view. Signing out of the Microsoft accounts will also sign the user out from the tree view: