Support templating for alerts

[matheuscscp]

For the Alert and Provider APIs general availability coming in Flux 2.10 mid-Q4, we should support some form of templating. Today, the message is composed with a special recipe for each Provider type. This has the advantage of ensuring alerts look good on the other side, as they have a fixed, well-tested form. However, this is also very limiting. We should introduce API fields for allowing Flux users to define custom templates for their alerts.

[ruslan-shaydullin]

Hi, I’d like to take on the design/RFC work for this. Before starting any implementation, I’d first work with the maintainers to agree on the API and provider semantics. Is this still available, and would Matheus or another maintainer be willing to sponsor the RFC? I’m happy to compare CEL with restricted Go templates in a Discussion or at a development meeting. Should the initial RFC cover every provider family, or would a staged rollout be acceptable? I’d also appreciate guidance on how this work should be sequenced relative to Alert v1 and Provider v1.

[matheuscscp]

Our plan is to use Go templates like the image-automation-controller message template. This issue is blocked by merging the upgrade to Events API v1 which @adri1197 is working on