Add ability to cleanup Configuration related disposables

From @JanEggers on Friday, 28 September 2018 06:43:27

Im having issues in a Unittests that complains that not all threads are ended when the unittest is completed.

Turns out this is due to using this:

 var configuration = new ConfigurationBuilder()
                .AddJsonFile("appsettings.json")
                .AddJsonFile(settingsPath, optional: true, reloadOnChange: true)
                .Build();

the reloadable json file generates Filewatchers that hang in a sleep if the test is ended.

the core problem is that there PhsicalFileProvider is disposable but currently I have to do something like this to clean it up:

foreach (var provider in config.Providers.OfType<JsonConfigurationProvider>())
            {
                if (provider.Source.FileProvider is PhysicalFileProvider pfp)
                {
                    pfp.Dispose();
                }
            }

I see two options here to improve the situation:

1. make IConfigurationRoot and all related classes disposable so I can just call config.Dispose();

2. resolve providers from DI so DI takes care of disposing the resources that require to do so.

maybe related to #3110

Copied from original issue: aspnet/Home#3564

[davidfowl]

Yes we need to support disposable configuration providers which is viral so the root will need to support it as well.

[analogrelay]

Closing based on the previous comment as part of backlog clean-up.