[Fleet] Add Processor Setting at the agent policy level

[gizas]

Describe the feature:

In the Managed Fleet Agent installation scenario we would like to add the possibility to users to be able to add the global processor configuration.

Describe a specific use case for the feature:

In more details we would like upon Kubernetes installation to be able to add processors like add_kubernetes_metadata or add_fields etc,
See the following block example per dataset or per policy level

 - add_kubernetes_metadata:
      default_indexers.enabled: false
      default_matchers.enabled: false
      add_resource_metadata:
              namespace:
                include_labels: ["namespacelabel1"]
                #labels.dedot: true
                #annotations.dedot: true
              node:
                include_labels: ["nodelabel2"]
                include_annotations: ["nodeannotation1"]
                #labels.dedot: true
                #annotations.dedot: true
              deployment: false
              cronjob: false

We know that there is already relevant discussion line #122892 or
inheritting processors by default like here or general support through custom pipeline usage since 8.4.1 (#133740), but we would like to offer this functionality to our K8s users as we believe this will enable extra possibilities.

• Can we somehow enable specific processors only to relevant integrations?
  • Can we have a drop-down list from globally inherited processors list and add those in the policy level? For eg. from a drop down list we can pick add_fields processor which will have default config like here
  • But add_kubernetes_metadata processor to be able to be chosen only when we are inside Kubernetes integration

cc @ruflin , @mlunadia , @tetianakravchenko

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)