Let Agents explizite target workflows by labels

[6543]

ontop of #4141:

 ### `WOODPECKER_AGENT_LABELS`
 
 > Default: empty
 
 Configures custom labels for the agent, to let workflows filter by it.
 Use a list of key-value pairs like `key=value,second-key=*`. `*` can be used as a wildcard.
+If you use `!` as key prefix it is mandatory for the workflow to have that label set (without !) set and matched.
 By default, agents provide three additional labels `platform=os/arch`, `hostname=my-agent` and `repo=*` which can be overwritten if needed.
 To learn how labels work, check out the [pipeline syntax page](../20-usage/20-workflow-syntax.md#labels).

Originally posted by @6543 in #3483 (comment)

[6543]

currently agents advertise there capabilities via labels ... but can not make sure to only get worklows that actually require them

this proposes an extention to have that option

[zc-devs]

Why not to implement it like in Kubernetes?

https://kubernetes.io/docs/tasks/configure-pod-container/assign-pods-nodes/
https://kubernetes.io/docs/concepts/overview/working-with-objects/labels/