feat: support http proxy in TwilioHttpClient

[thehackercat]

Ref #409

Adding support for TwilioHttpClient to make requests behind a custom proxy, for example

from twilio.http.http_client import TwilioHttpClient
from twilio.rest import Client

sid = ''
token = ''
http_client = TwilioHttpClient(proxy={'http': 'http://proxy.twilio.com:8118'})
twilio_client = Client(sid, token, http_client=http_client)
twilio_client.messages.create()

Contributing to Twilio

All third-party contributors acknowledge that any contributions they provide will be made under the same open-source license that the open-source project is provided under.

• I acknowledge that all my contributions will be made under the project's license.

[childish-sambino]

👍