Optimization for Azure Front Door watch requests

[avanigupta]

Use HEAD requests during refresh when checking for changes. We only need ETag and x-ms-date headers to decide if a page has changed or not. So we can reduce bandwidth usage for watch requests by making HEAD requests instead of GET. When change is detected, proceed to reload configuration with GET requests as usual.

@zhiyuanliang-ms @drago-draganov @jimmyca15

[zhiyuanliang-ms]

This feature requires SDK change. I think current App Config SDK doesn't have an API to send HEAD request.

[zhiyuanliang-ms]

Did some investigations on .NET and JS SDKs

1. SDK has already generated code for the HEAD request based on our REST api: JS SDK and .NET SDK

2. Azure Storage SDK has public API which sends HEAD request.

So, it is quite straightforward, we can add a new public method to send HEAD request to check key values for ConfigurationClient.

cc @zhenlan @linglingye001