[Feature] IProvider.GetTokenAsync should accept additional scopes

[shweaver-MSFT]

Describe the problem this feature would solve

Currently, the only way to add scopes is to configure them during provider initialization. This means that it is difficult to add any additional scopes at runtime without completely reconfiguring the global IProvider instance.

Describe the solution

Enable IProvider.GetTokenAsync to accept an array of access scopes:

IProvider provider = ProviderManager.Instance.GlobalProvider;
string token = provider.GetTokenAsync(scopes: new string[] { "Tasks.ReadWrite" }, silentOnly: false);

Providing none will default to any pre-configured scopes in the provider.

Describe alternatives you've considered

None

Additional context & Screenshots

This request is inspired by the pattern in MGT.

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