Add alerts/watchers as assets in packages

[ChrsMark]

At the moment we don't support alerts and watchers to be part of a package.
At elastic/integrations#4997 we explore how the alerting feature can enhance our Kubernetes solution.
After having the first bunch of alerts/watchers we decided to include them in the docs' section for now with elastic/integrations#5364.

The purpose of this issue is to propose to provide the option for alerts/watchers to be installed when installing the package. These means we should add them to their own place in the package-spec, and we should have a follow-up task in Kibana to install them when available.

[jsoriano]

@ChrsMark what would be needed to store in the packages, only the watcher definition? Or would some additional setting or identifier be needed too?

[ChrsMark]

@jsoriano based on what we have so far, watcher's definitions would be the only thing to be stored. I cannot think of anything else what would need to be stored 🤔

[ruflin]

Instead of adding support for watcher, we should instead add support for alerts and slo.

[jsoriano]

Closing as duplicate of #121, let's keep discussions about this there.