Migrate google/btree to k8s util

[pjsharath28]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

The github.com/google/btree library used by k8s and etcd has been archived and is no longer maintained. This raises long-term concerns around security fixes and CVEs.

We discussed multiple options to address this in the issue. Given that we only rely on a single source file from the library, we concluded that copying the code into k8s.io/utils is the simplest option. This avoids introducing or maintaining a separate fork while preserving identical behaviour.

Which issue(s) this PR is related to:

Discussion issue: etcd-io/etcd#20991

Special notes for your reviewer:

Upstream Google attribution is preserved and the file header follows Kubernetes conventions while remaining Apache-2.0 compliant.

Does this PR introduce a user-facing change?

NONE

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

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[aojea]

/assign @dims

side note @dims, it seems the github actions are waiting for manual approval, you can change that option in in the repo settings IIRC so they always run

[k8s-ci-robot]

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[skitt]

This isn’t relevant and should be removed.

[skitt]

k/k only uses the generic implementation, do we need to keep both in utils?