Add documentation how to set up custom per request HttpMessageHandlers

We have an endpoint that routes traffic to various endpoints that use different credentials and auth types (username/password combinations; basic and digest auth) based on the request parameters.

For now we have setup a factory class that instantiates a new HttpClient and HttpMessageHandler implementation for every request that is passing through. That does not seem right. We dispose the HttpClient in the consumer but is the HttpMessageHandler disposed correctly automatically?

Ideally we would leverage a pooled HttpMessageHandler approach such as is mentioned in this documentation. How can we add our own HttpMessageHandlers dynamically?

Also see the Stack Overflow question we have raised:
https://stackoverflow.com/questions/67588449/httpclient-pattern-with-dynamic-username-and-password

```c#
public class AuthenticatedHttpClientFactory : IAuthenticatedHttpClientFactory
{
public HttpClient GetClient(string username, string password)
{
var messageHandler = new HttpClientHandler()
{
Credentials = new NetworkCredential(username, password)
};
var client = new HttpClient(messageHandler);
return client;
}
}
```

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#### Document Details

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* ID: 7b506236-b211-eb9d-b571-5efecdde76c8
* Version Independent ID: 24434b0e-0b82-e477-9440-2cc8f135df75
* Content: [Use IHttpClientFactory to implement resilient HTTP requests](https://docs.microsoft.com/en-us/dotnet/architecture/microservices/implement-resilient-applications/use-httpclientfactory-to-implement-resilient-http-requests)
* Content Source: [docs/architecture/microservices/implement-resilient-applications/use-httpclientfactory-to-implement-resilient-http-requests.md](https://github.com/dotnet/docs/blob/main/docs/architecture/microservices/implement-resilient-applications/use-httpclientfactory-to-implement-resilient-http-requests.md)
* Product: **dotnet-architecture**
* Technology: **microservices**
* GitHub Login: @nishanil
* Microsoft Alias: **nanil**