opentelemetry-exporter-http-transport: enable entry-point loading of transport implementations (#5320)

* opentelemetry-exporter-http-transport: enable entry-point loading of transport implementations

* add changelog fragment

* fix Python 3.10 typing import error

* fix typecheck errors

* check that returned class is a subclass of BaseHTTPTransport

* Change kwargs type hint to Any in _requests.py

* Refactor kwargs type hint to Any for urllib3

* address comments