Allow different agent configurations per Cluster

[anmazzotti]

This is a feature request to make the fleet-agent configuration more granular.
For example on GKE the fleet-agent metrics port 8080 is already bound when also running the agent in hostNetwork: true mode.

A user can edit the FleetAddonConfig.spec.cluster.agentEnvVars to configure a different port, however this will apply to all other Clusters.

I'm not entirely sure how this feature can be implemented.
Two ideas are:

1. Simply make FleetAddonConfig.spec.cluster a list. Different selectors will apply different configurations.
2. Use special annotations on the CAPI Cluster to override configuration. (however this is going to be awkward to map things like agentEnvVars.