TokenAcquirerFactory.GetDefaultInstance() By default reads an appsettings.json, why not azure app config

- [ ] bug report -> please search issues before submitting
- [x ] feature request
- [ ] documentation issue or request
- [ ] regression (a behavior that used to work and stopped in a new release)

```
// Get the Token acquirer factory instance. By default it reads an appsettings.json
// file if it exists in the same folder as the app (make sure that the
// "Copy to Output Directory" property of the appsettings.json file is "Copy if newer").
var tokenAcquirerFactory = TokenAcquirerFactory.GetDefaultInstance();
```

is it possible to somehow add functionality to get the value from your own azure app configuration the same way like below example so I dont need to store client secret inside the app, by default GetDefaultInstance(); gets that from appsettings.json that is not safe for the production
```
tokenAcquirerFactory.Services.AddDownstreamApi(ServiceName,
config.GetSection("DownstreamApi"));
```