Feature Request: http interceptor in httpx

[hesch]

Feature:

Implement a registration for http request interceptors, that can modify the request or do some other logic before the request is executed.
This can be used to set Headers dynamically or do necessary preflight requests.

Use Case:

I am currently implementing load tests that need authn/authz for an OAuth 2 secured endpoint. Everything works as expected, but if the load test is running longer than the lifetime of the access token, requests start to fail. At the moment, I am solving this by having a wrapper around httpx that checks/refreshes access tokens before firing requests.