feat(notifications): added Discord notifications (#52)

* feat(notifications): added Discord notifications

* refactor(notifications): interface to implement BuildField

* feat(notifications): option to skip sending notifications for empty runs

* feat(notifications): progress and runtime in footer

* feat(notifications): add removal reason

* feat(notifications): change format to one embed per event

* feat(notifications): add discord rate limiting and escape messages

* feat(notifications): client in footer and small refactor

* feat(discord): support custom username and avatar

* feat(notifications): update readme & don't send summary with only one embed

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Co-authored-by: salty <salty@saltbox.dev>
Co-authored-by: soup <s0up4200@pm.me>