Make it easier to provide a custom HttpClient

[atrauzzi]

In some instances, I may wish to provide my own HttpClient.

Using the dapr scenario linked above as an example, the HttpClient obtained will be pre-configured to handle communication between my application and FusionAuth. This also means that I could be setting headers at the dapr resource level, rather than supplying it from my application and don't need the FusionAuth SDK to handle auth for me.

Unfortunately though, configuring the HttpClient is hidden under an inheritance hierarchy and isn't as extensible as it could be.
That said, even if I went to the effort of implementing this (very JVM-esque 😜) object+factory, I wouldn't be able to subclass the default implementations because they're marked as private.

I think there are some extensibility scenarios here that could be improved so that people can benefit from nice strongly typed access to FusionAuth APIs through its SDK 🙂