[Alert grouping] Add tags as default grouping field

[maryam-saeidi]

Summary

In the previous ticket, we added rules and sources as default filters. In this ticket, we would also like to add tags. The current grouping implementations seem not to support grouping by tags separately, but it will concat all the tags by comma and then group them accordingly, which does not match what we want.

Acceptance criteria

• Add tags as a default group with the ability to group alerts for each tag separately

[elasticmachine]

Pinging @elastic/obs-ux-management-team (Team:obs-ux-management)