Please add support for PAC type proxy and ntlm authorization

[Testetsun-happy]

Currently sdk only support fixed IP proxy and username/pwd authorization.
however in some global etnerprise, use a dynamic PAC list as proxy setting.
so please add support for this scenario, thanks.

sample code:
from requests_ntlm3 import HttpNtlmAuth
from pypac import PACSession, get_pac

auth=HttpNtlmAuth("proxyUserName","proxyPassword")
pac = get_pac(url="http://pacserver.mycompany.com/config.pac")
s = PACSession(pac)
s.auth =auth
r = s.post(config["http://a.com/sample/,json =pload)

[l-iberty]

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