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feat(manifests): Support high availability for the controller manager in the Helm chart

Add four opt-in `manager` values so the controller manager can run
highly available: podDisruptionBudget (disabled by default, fails the
template if enabled with replicas < 2), topologySpreadConstraints
(empty by default), priorityClassName (empty by default, no
PriorityClass is created), and a soft preferred podAntiAffinity
shipped as the new default for affinity.

Leader election already allows multiple replicas, but the webhooks
default to failurePolicy: Fail, so any gap with zero Ready manager
pods rejects every TrainJob/TrainingRuntime create and update. These
values close that gap without changing behavior for existing
installs: `helm template` on this branch vs. upstream/master, same
values, differs only in the new affinity default, which is a soft
scheduling preference and never blocks a pod from being scheduled.

Verified webhook cert management holds up at replicas=2 on a 3-node
kind cluster, including after force-deleting the leader pod mid-flight:
caBundle stayed in sync with the cert Secret throughout, and TrainJob
admission had no gap.

Fixes #4083

Signed-off-by: myselfkunal <kunalshawip@gmail.com>