Add support to load configuration from snapshots

[zhenlan]

API proposal:

public AzureAppConfigurationOptions Select(string snapshotName)

cc: @avanigupta @jimmyca15 @drago-draganov

[jimmyca15]

That doesn't conflict with our existing API

public AzureAppConfigurationOptions Select(string keyFilter, string labelFilter = LabelFilter.Null) ?

[zhenlan]

Good catch. It will. How about

public AzureAppConfigurationOptions SelectSnapshot(string name)

[jimmyca15]

Sounds good to me.