Support snapshot reference

[zhiyuanliang-ms]

Snapshot Reference

A snapshot reference is a configuration setting that references a snapshot in the same App Configuration store. When loaded, the provider resolves it and adds all key-values from that snapshot. Using snapshot references enables switching between snapshots at runtime, unlike using a snapshot selector, which requires code changes and/or restarts to switch to a new snapshot.

The content type of a snapshot reference is: "application/json; profile=\"https://azconfig.io/mime-profiles/snapshot-ref\"; charset=utf-8";

Behavior difference from .NET provider

Different from PR #689

The snapshot api design in JS provider is different from .NET provider.
In JS provider, keyvalue and feature flag are fully decoupled. The load and refresh operations for key values and feature flags are performed separatedly. The selector for key value will not load feature flags, vice versa. You must explicitly specify selectors for them.

To load key value or feature flag from snapshot, you must use a snapshot selector i.e. SettingSelector.snapshotName. When you specify a snapshot selector for key values, even if there is any feature flag in that snapshot, the feature flag will not be loaded when loading key values. You must also specify the same snapshot selector in FeatureFlagOptions.

It is a library level choice. We won't break it. So, when resolving a snapshot reference, feature flags in the snapshot will be ignored.

Note: This PR is not ready for review yet. It depends on the Azure JS SDK adding support for snapshot reference types. After that update, I’ll replace all existing helper functions with the SDK ones.

[zhiyuanliang-ms]

Should we handle the case that sentinel key is a snapshot reference?