feat: Add avatar_url override for webhook struct, resolves #1416

[Enovale]

This is a fix for #1416, adding a new property to the webhook struct to allow for custom avatar urls.

If an avatar_url is set, it will be used instead of the custom uploaded avatar.

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• My code follows the coding style guide.
• I tested that my change works before raising the PR.
• I have ensured that I did not break any existing API calls.
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[braindigitalis]

just a couple of minor comment changes, and this is good to merge!
Thanks for your contribution!