Add mastodon as a notification channel

[developer-guy]

Mastodon is free and open-source software for running self-hosted social networking services. There is a Go SDK of the Mastodon API¹, so we can add Mastodon as a new notification channel.

Similar work has been done in the GoReleaser project².

/cc @Dentrax

Footnotes

1. https://github.com/mattn/go-mastodon ↩

2. https://github.com/goreleaser/goreleaser/pull/3567 ↩

[xchose]

Hi, could you assign this to me? I've added the Zoom (#1349) and incident.io (#1353) providers previously and have a working implementation for Mastodon (plain HTTP via POST /api/v1/statuses, no SDK dependency), e2e-tested on a kind cluster against a real mastodon.social account. PR incoming.