Add mastodon as new notification channel

# Summary

[Mastodon](https://joinmastodon.org) is free and open-source software for running self-hosted social networking services.

# Motivation

Mastodon will become one of the leading social networks in the industry. So, there is considerable interest in moving from Twitter to Mastodon.

# Proposal

There is a Go SDK of the Mastodon API[^1], so we can add Mastodon as a new notification channel.

Similar work has been done in the GoReleaser project[^2].

[^1]: https://github.com/mattn/go-mastodon
[^2]: https://github.com/goreleaser/goreleaser/pull/3567

/cc @dentrax

Start by reading the notifications-engine's existing notification channels and the go-mastodon SDK linked in the issue; the repository does not identify a specific file or test to begin with. Compare the related GoReleaser pull request, then define done as delivering notifications through Mastodon with coverage consistent with the project's existing channels.

Written by the indexing model from the issue text.