Auth v6: No way of clearing the subscription cache without reloading the window

[bwateratmsft]

The registries view for Azure has a minor issue--if subscriptions change or an account is signed out, there isn't a way to clear the auth provider's cache. We should add this.

vscode-containers/src/tree/registries/Azure/AzureRegistryDataProvider.ts

Line 65 in 1cc5f7a

const subscriptions = await this.subscriptionProvider.getAvailableSubscriptions();

[bwateratmsft]

Similar to what we do in microsoft/vscode-azureresourcegroups#1292, we should initiate a cache clear only on user-initiated manual refresh.