vcsim: support alarm definitions and trigger on datacenter, cluster and host

[rikatz]

Is your feature request related to a problem? Please describe.
We are doing some integration tests with vcsim, and some actions are conditional to existing CPU and memory alarms on hosts and clusters.

This way, the idea is to make vcsim support the definition of alarms on hosts, clusters and datacenters and also support triggering it.

Initially there is no desire to make a "top/bottom" alarm definition, where the alarm goes to a datacenter and flows all the path down to the host (we can do it granular)

Describe the solution you'd like
Allowing the definition of alarms on datacenter, clusters and hosts.
Allowing triggering these alarms on the objects

Describe alternatives you've considered
N/A

Additional context
Add any other context or screenshots about the feature request here.