Add node management APIs: drain/undrain, taint/untaint, labels and automatic workload relocation

[miladhzzzz]

Motivation

• Provide operator-facing control-plane APIs to manage node lifecycle (drain/undrain), taints, and labels so nodes can be taken out of scheduling without manual etcd edits or relying on heartbeat-only flags.
• Ensure scheduler enforces taints and drains by excluding tainted/draining nodes from placement and relocating workloads automatically when an operator drains a node.
• Surface node taints and label changes through the control API so the gateway and clients can observe node metadata consistently.

Description

• Added new control RPCs to the scheduler AgentControl API and proto: DrainNode, UndrainNode, TaintNode, UntaintNode, SetNodeLabel, and DeleteNodeLabel, plus message types for taints, drain responses, and label operations in api/proto/control.proto.
• Persisted taints on the scheduler Node model and exposed them in NodeView, plus regenerated protobuf/gRPC stubs used by scheduler, gateway, and shared pkg clients.
• Implemented scheduler-side node management methods (MarkNodeDraining, MarkNodeReady, TaintNode, UntaintNode, SetNodeLabel, DeleteNodeLabel) and an updateNode helper that persists changes to etcd and updates the in-memory cache.
• Preserved Draining status during heartbeats and made heartbeat responses set drain_node=true when the node is in Draining state.
• Excluded nodes with NoSchedule-style taints from selectNodeForWorkload() and added RelocateWorkloadsFromNode() which automatically reassigns workloads when a node is marked Draining.
• Added gRPC handlers in the scheduler (internal/grpcapi/service.go) and gateway proxy methods/controllers (persys-gateway/services/*, controllers/*, routes/*) plus HTTP routes and JSON handlers for the new operations.

Testing

• Generated updated protobuf stubs with make proto in persys-scheduler and persys-gateway and regenerated dependent internal/controlv1 and pkg/scheduler/controlv1 files successfully.
• Ran unit/test targets: cd persys-scheduler && go test ./internal/controlv1 and cd persys-gateway && go test ./internal/controlv1, both completed without errors.
• Exercised additional package test runs (no-test-files suites) via go test across internal/grpcapi, internal/scheduler, and gateway controller/service/routes packages which returned successfully (no failing tests).
• Verified gofmt and git diff --check validations as part of the change workflow with no formatting or lint errors reported.

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[miladhzzzz]

/LGTM