Add a new API to allow custom client options

[zhenlan]

Proposed API

public AzureAppConfigurationOptions ConfigureClientOptions(Action<ConfigurationClientOptions> options)

This API should allow a user to overwirte the properties of ConfigurationClientOptions we use by default.

For example, in the case of Azure/AppConfiguration#179, the customer can do

using HttpClient client = new HttpClient(
    new HttpClientHandler()
    {
        Proxy = new WebProxy(new Uri("http://example.com"))
    });

builder.AddAzureAppConfiguration(o =>
    o.Connect(...)
     .ConfigureClientOptions(co => co.Transport = new HttpClientTransport(client));
)

Reference here.

cc: @drago-draganov @jimmyca15

[abhilasharora]

This feature has been released as a part of version 3.0.0 of the package Microsoft.Extensions.Configuration.AzureAppConfiguration.