Add proxy support

[370rokas]

Adds support for using proxies.

Example usage:

from pysteamauth.auth import Steam

async def main():
    steam = Steam(
        login='login', 
        password='password',
        proxy="http://ip:port"
    )
    
    await steam.login_to_steam()

[370rokas]

@sometastycake any possibilities to merge this soon into master?

[m-a-prod]

Merge this pls