Add support for keys with labels

[kylehayes]

I'm interested in providing a contribution to add support for keys with labels (https://docs.microsoft.com/en-us/azure/azure-app-configuration/concept-key-value#label-keys).

My thought was adding logic to loadFromAzure that would look for a new env var called AZURE_APP_CONFIG_LABELS. This value would be passed to getAppConfigurations. In getAppConfigurations, the labels would be passed to client.listConfigurationSettings into the labelFilter key for the ListConfigurationSettingsOptions property.

[danielfsousa]

That would be great! Feel free to open a PR