fix: pass proxy_url to aiohttp request in AsyncTwilioHttpClient

[lariossoler]

Resolves #788

Hello!

This PR addresses the issue where AsyncTwilioHttpClient receives a proxy_url during initialization but ignores it when constructing the actual aiohttp request kwargs.

By injecting the proxy into kwargs, we restore the ability to use the async client behind corporate firewalls.

Fixes

A short description of what this PR does.

Checklist

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