Add support for notifying into matrix

[telmich]

Matrix (matrix.org) is an open source federated chat is more and more used as a replacement for Slack.
Matrix has a very simple API that is alread implemented by services such as gitea.

The typical parameters for a matrix webhook are:

• Homeserver URL
• Room ID (where to post)
• Access token (authentication)
• Event type (optional, can be m.notice or m.text)

[stefanprodan]

You could use the Slack provider along with https://matrix.org/docs/projects/as/matrix-appservice-webhooks

[telmich]

That is absolutely correct, @stefanprodan. However that would mean everyone using Matrix will need to add their own webhook server. That's why I was wondering if it makes sense to add this functionality directly into flux.

[stefanprodan]

@telmich I'm not against implementing an alert provider for Matrix, I just wanted to add a note here for others, that there is a workaround for using Matrix even if Flux don't supports yet.