Feature Request: Unescaping characters in summary

Requirement

The only user controlled input for forwarded alerts currently is alert.summary as far as I'm aware. To allow the customisation of alert messages in Matrix, a capability for adding line breaks, bold text, etc. would be great.

Grafana allows control characters, but escapes all HTML characters in the webhook sent. Matrix on the other hand does not allow control characters but allows HTML.

Value proposition

Allow for very customized alert messages without touching large amounts of code here or introducing new systems.

Design ideas

A possible solution would be unescape the summary block via a typed string. This could also be enabled through a cli option but should be safe as default since matrixs org.matrix.custom.html message type should only support safe tags anyway.