Configure existing HttpClient using WebApplicationFactory (e.g. for named clients using IServiceCollection.AddHttpClient<T>)

### Is your feature request related to a problem? Please describe.

This is a duplicate of #5967 but because it was closed due to inactivity, I am creating a new one with more details and use cases.

I am trying to run tests against a Hot Chocolate (GraphQL) server and the Strawberry Shake GraphQL client, which uses `HttpClientFactory` with a named client.

I'd like to start my app in my tests using `Microsoft.AspNetCore.Mvc.Testing`, _and_ use the code generation features of Strawberry Shake to generate the GraphQL client. I can't, because Strawberry Shake uses the `HttpClientFactory`, and all of the methods required to build _or_ configure an HttpClient are internal in `WebApplicationFactory`.

Example of the code in my tests I'd like to have (simplified and inlined):

```c#
var services = new ServiceCollection();
// Here I cannot easily configure the client since it needs a custom HttpMessageHandler from TestServer
services
.AddHttpClient(GraphClient.ClientName, client => client.BaseAddress = _factory.Server.BaseAddress)
.ConfigurePrimaryHttpMessageHandler(_ => _factory.Server.CreateHandler());
services
// This is a library that depends on HttpClientFactory
.AddGraphClient()
// This is using HttpClientFactory internally
var graphClient = services.BuildServiceProvider().GetRequiredService();
```

### Describe the solution you'd like

It would be much simpler if instead of just having `CreateClient()`, I could also use `ConfigureClient(HttpClient)`.

e.g.:

```c#
.ConfigureHttpClient(client => _factory.ConfigureClient(client));
```

The code is mostly the same as is already inside `WebApplicationFactory`, however instead of making more properties and methods public, there would be a method to assist in cases where using HttpClientFactory is desirable.

### Additional context

While my use case (and the one from #5967) are because of external libraries, I believe making use of a pooled HttpClient factory can make sense for reusing existing code from internal libraries.