Add StartupOptions for time-based retries on startup

[amerjusupovic]

StartupOptions and the Timeout property

This PR adds the new Startup property to AzureAppConfigurationOptions. You can set the Startup.Timeout property equal to a TimeSpan that represents how long the provider should continue retrying the store and replicas passed to the provider before failing.

Example usage:

config.AddAzureAppConfiguration(options =>
{
   // . . .
    options.ConfigureStartupOptions(startupOptions =>
    {
        startupOptions.Timeout = TimeSpan.FromMinutes(5);
    });
});

[zhenlan]

                if (_mappedData == null)

Is this only triggered during refresh? If so, it means the startup is already done (though initial configuration load had failed). We shouldn't use the startup logic here, because if anything fails here it will be tried again in the next refresh. #Closed

---

Refers to: src/Microsoft.Extensions.Configuration.AzureAppConfiguration/AzureAppConfigurationProvider.cs:211 in c8ada35. [](commit_id = c8ada35504eb98d64cbb06ed008a2354f58eadf7, deletion_comment = False)

[jimmyca15]

pr example usage needs an update.

[jimmyca15]

@amerjusupovic will you be updating the target branch to preview?