Add design document for the HAProxy control plane

Igorrequested to merge
haproxy-control-plane-design-doc into main

Why is this change being made?

Adds a design document for the HAProxy control plane: desired server state in Consul KV, applied on every GitLab.com load balancer through HAProxy's agent-check, replacing the per-node TCP admin socket that ChatOps, set-server-state, and ha-ctl used to drive.

The work is done (production-engineering#29645), so this is written after the fact from the proposal and the record on that issue. The proposal is kept as written, with Outcome notes where the implementation departed from it (per-row plus group KV keys, no gradual undrain, rollout by backend instead of by node, ha-ctl removed rather than converted, the HAProxy state-file agent-port bug). Modeled on the Runner Managers on Kubernetes document so the two infrastructure design docs read the same way.

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