Add proxy variable

[jhqwqmc]

Summary

Add the use of the proxy variable to the from_url function of the webhook class.

During use, I found that the from_url function did not provide a proxy parameter to pass in. My area requires a proxy to access discord, so I added this parameter.

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)

[No767]

If you were to ask me, this isn't needed. I would wonder why even support proxies for webhooks?

[Rapptz]

The reason for this is because you can set the proxies on the ClientSession you create instead.

[jhqwqmc]

ClientSession does not provide proxy variables. You need to pass proxy variables to Webhook.

[jhqwqmc]

Needing a proxy is also a helpless move. Our country has blocked the website discord.com.

[Rapptz]

Actually it seems you're right, I forgot this change happened to aiohttp a while back. This PR is insufficient though. It needs to support proxy_auth parameter as well and be properly documented.

[jhqwqmc]

I just added proxy_auth

[Rapptz]

This is still missing documentation, and most likely needs to run black.