Support listing by etags

[Eskibear]

          Did JS SDK release collection monitoring API? I see they had a PR for this but not sure if this exists in the latest stable release: https://github.com/Azure/azure-sdk-for-js/pull/28555

Originally posted by @avanigupta in #65 (comment)

SDK v1.6.0 includes the new API listing by page etags. We might adopt that in refreshing both key-values and feature flags.

[Eskibear]

feature flags are refreshed based on page etags in #65

[Eskibear]

For key-value refreshing, the behavior is related to watchedSettings

Current behavior when watchedSettings is

• undefined: Error
• empty list: Error <--- // To be enhanced
• non-empty list: sentinel-based refresh.

With the pageEtag based listing API. Now we are able to impl refresh without requiring a sentinel key (watched setting). The proposal is, when watchedSettings is an empty list, it indicates we don't watch any specific setting, but use page Etags to determine whether to refresh or not.

[zhiyuanliang-ms]

#133