Agents: Self Registration - support labels/annotations

[drewbailey]

Problem

The https://argo-cd.readthedocs.io/en/stable/operator-manual/applicationset/Generators-Cluster/ discovers target clusters by reading Argo CD cluster secrets and filtering them by labels (e.g. env=production, region=us-west-2). With self registering agents in argocd-agent, cluster secrets are created by the principal during self-registration, but there is no way for an agent to declare labels/annotations to add to the cluster secret.

A clear and concise description of what you want to happen.

Agent side (cmd/argocd-agent/agent.go, agent/options.go)

• Add --cluster-labels key=val,key=val and --cluster-annotations key=val,key=val flags
• Store as map[string]strings on agent options
• Send as part of the existing connect/auth handshake (extend the auth request proto, or a new dedicated RPC— see tradeoffs below)

Wire protocol two potential options:

1. Extend the existing AuthRequest proto to carry cluster_labels / cluster_annotations map fields
2. New RegisterAgent RPC called after auth

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
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