Backport to 1.34: Making CapacityBuffer ProvisioningStrategy a string (not enum)

[jakubin]

What type of PR is this?

/kind cleanup
/kind api-change

What this PR does / why we need it:

Backport #8832 to cluster-autoscaler 1.34

Which issue(s) this PR fixes:

Different providers can have their own custom provisioning strategies. This field cannot be constrained by known in advance enum values.

Special notes for your reviewer:

Does this PR introduce a user-facing change?

Make CapacityBuffer ProvisioningStrategy a string

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

[k8s-ci-robot]

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[x13n]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jakubin, x13n

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