Add request lifecycle hooks aka middleware

[TheCaffinatedDeveloper]

I saw from an earlier issue that it was deemed token refresh is out of scope for this library.

While disappointing, I can get behind the logic but request that the framework then expose hooks so that we can provide token management ourselves.

For example, right now I am subclassing AsyncFHIRClient and overriding _do_request so that I can catch expired tokens and handle refreshing. This isn't ideal since I am dependent on this library's implementation which is subject to change. It would be nice if instead, there were a before_request hook that allows us to mutate request headers and the like accordingly.

BTW, this is by FAR the best implementation of a FHIR client I have come across. While aimed mostly towards Aidbox, it has been working fine with HAPI and is leaps and bounds above the SMART on FHIR client that is somehow advertised excessively.