Add an option to enable load balancing between replicas

[amerjusupovic]

https://github.com/Azure/AppConfiguration/issues/868

Adds the following new API to enable load-balancing of requests between replicas for an App Configuration store:

config.AddAzureAppConfiguration(options => {
    // ...
    options.LoadBalancingEnabled = true;
});

Load balancing will always operate on all endpoints available to the provider. If the ReplicaDiscoveryEnabled property is set to true and endpoints have also been passed in explicitly like with AzureAppConfigurationOptions.Connect, then the provider will treat the passed-in endpoints as highest priority for sending requests. It will still load balance between passed-in endpoints and discovered replicas over time.

[jimmyca15]

Discussed offline, we can revert the last commit since we already depend on repeatable enumeration here.