Custom webhook feedback URL

[philpax]

As requested by a few people - a way to deliver feedback to an endpoint owned by a plugin dev in a variety of formats, so that they don't have to look at #feedback to get their plugin's feedback specifically and so that they can send feedback to their own bug reporting system.

Requirements:

• Be able to disable feedback entirely, deliver feedback to #feedback, deliver feedback to the specified custom endpoint, or deliver feedback to both. (i.e. all four variations)
• Must not leak the endpoint URL to anyone. This will likely be done by encrypting it, as we do with the secrets: https://goatcorp.github.io/plogon-secrets/
• Must be able to handle standard payload formats. Probably just Discord for now?

Given that, the solution will probably:

• Store the custom endpoint in the manifest, as well as instructions on how to deliver it. I'd say start with just a basic JSON payload ({author: string, message: string}) and a Discord payload. We can add more if people suggest them.
• Bugbait decrypts the payload with its private key, and then delivers the payload to the endpoint, in addition to Discord if specified

[philpax]

Another option that would be nice to have: getting pinged by your Discord username. We could let people specify their username in the manifest, and then have an option to be pinged.