opentelemetry-instrumentation-httpx: add ability to capture custom headers (#4047)

* opentelemetry-instrumentation-httpx: add ability to capture custom headers

* update CHANGELOG.md

* move optional args to end of arg list for http helper methods

* small fix

---------

Co-authored-by: Riccardo Magliocchetti <riccardo.magliocchetti@gmail.com>

Author: herin049

CHANGELOG.md

@@ -27,6 +27,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   ([#3938](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/3938))
 - `opentelemetry-instrumentation-aiohttp-server`: Support passing `TracerProvider` when instrumenting.
   ([#3819](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/3819))
+- `opentelemetry-instrumentation-httpx`: add ability to capture custom headers 
+  ([#4047](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/4047))
 
 ### Fixed