Improved Silence management

[QuentinBisson]

User Story

• As a Giant Swarm engineer, I want to be able to manage silences using Silence CRD only (without having to access to alertmanager)
• As a Giant Swarm customer, I want to be able to see the silences on my installation, maybe create them for temporary test clusters as to not page Giant Swarm engineers

Tasks

### Tasks
- [ ] https://github.com/giantswarm/giantswarm/issues/25312
- [ ] https://github.com/giantswarm/roadmap/issues/1045
- [ ] https://github.com/giantswarm/giantswarm/issues/26485
- [ ] https://github.com/giantswarm/roadmap/issues/1043
- [ ] https://github.com/giantswarm/giantswarm/issues/27553
- [ ] https://github.com/giantswarm/giantswarm/issues/30728
- [ ] Reflect status of silences in Alertmanager in Silence CRs (list of silenced alerts), maybe add an annotation on the cluster to know that it is affected by silences (kinda like what VPA does)
- [ ] Make alertmanager private if possible (port-forward only)?
- [ ] Push silence operator into prometheus-operator
- [ ] https://github.com/giantswarm/roadmap/issues/3729

[QuentinBisson]

cc @giantswarm/sig-opsprocess did I miss anything?

[JosephSalisbury]

looks good to me - i think the main thrust of the discussion was that we should have only one way to create silences, and that should be via silence CR, and i think this covers that