feat: added support for Discord Webhooks

[dannyhammer]

Extends the idea in this PR for producing a Discord message via webhook when a test completes.

To use, create a file named webhooks.json in the working directory of the project and format it like so:

{
    "USERNAME": "WEBHOOK"
}

where USERNAME is the OpenBench username of the person that wants to be notified and WEBHOOK is the Discord Webhook URL. When a test authored by USERNAME completes, it will send a Discord message to the appropriate channel with the test results.

You can specify multiple users and webhooks in the webhooks.json file. This is so that, on a shared OB instance, you can avoid being notified for tests that you did not author.

Example:

[dannyhammer]

In light of recent changes to my OB instance, I've made some changes to this PR and updated the content original post. Now, you can specify multiple webhooks and multiple users, allowing you to only have your webhook notified when a test that you authored completes.