Support dynamic refresh

[Eskibear]

Design

New APIs:

interface AzureAppConfiguration {
  ...
  refresh(): Promise<void>; // api to trigger refresh
  onRefresh(listener: () => any, thisArg?: any): void; // success callback
}

interface AzureAppConfigurationOptions { 
    ... 
    refreshOptions?: RefreshOptions; 
}

interface RefreshOptions {
  enabled: boolean; // default to false
  refreshIntervalInMs?: number; // default to 10s
  watchedSettings?: WatchedSetting[]
}

interface WatchedSetting  { 
    key: string; 
    label?: string;
}

Refresh Behavior: