Allow mutating alerts on pipeline stages

Currently the alertmanager notify stages allow things like dedupe, group by amongst other things. What we have observed is that mutating of alerts carry a lot of interesting possibilities such as:

• attach exemplars to each alert via annotations and paint them on the corresponding receiver
• group alerts by a certain grouping key and attach a root cause which can be fetched from a certain external API

currently alerts are dispatched parallel to each receiver which means that any stage is concurrently invoked causing map updates to annotations to cause a panic.

some of the things that would be useful:

• support mutation stages that are common across all receivers (we would hate to see different sample traces across various receivers)
• support grouping of alerts post mutation to facilitate more complex things

Currently things like AlertStoreCallback can be used to achieve some of this but said callback blocks the POST/PUT API which can cause a slow down on the rule manager.

Start by reading the alert notification pipeline and the AlertStoreCallback path, then trace how alerts are dispatched in parallel to receivers. Define how shared mutation stages, post-mutation grouping, and safe annotation updates should work; done means the design supports these behaviors without blocking the POST/PUT API or causing concurrent map-update panics.