[CASCL-1386] (8/11) Add node drain primitives and evict ASG nodes

[L3n41c]

What does this PR do?

Introduces the shared node-drain primitives — node cordoning (cordonNodes), pod eviction/draining (drainNode) and the pod-eligibility predicates (shouldSkipEviction …) — together with their first consumer, ASG eviction (cordon, drain, terminate the instances, lock the ASG to 0).

Motivation

PR #3026 is too large to review as a single change. This is part 8 of 11 of a stack that splits it into small, individually-reviewable pieces that each build and pass tests on their own. See #3026 for the full feature context and end-to-end QA.

Additional Notes

Stacked PR — the base branch is lenaic/CASCL-1386-evict-07-eks-mng, not main, so the diff shows only this step's change. The drain primitives are bundled with the ASG evictor on purpose: golangci-lint's unused linter rejects an unexported helper that has no caller in the same commit, so the primitives land in the same PR as the first evictor that calls them. Parts 9–10 reuse them. The code is taken verbatim from #3026 (rebased onto main); there are no logic changes versus #3026.

Minimum Agent Versions

N/A — kubectl-datadog plugin only.

Describe your test plan

go test ./cmd/kubectl-datadog/autoscaling/cluster/... passes at this commit. Full end-to-end QA is tracked on #3026.

Checklist

• PR has at least one valid label
• qa/skip-qa label applied (not independently shippable; QA tracked on [CASCL-1386] Add evict-legacy-nodes subcommand to drain non-Datadog node groups #3026)
• All commits are signed

[codecov-commenter]

Codecov Report

❌ Patch coverage is 89.69697% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 44.36%. Comparing base (f9c10d1) to head (0b04574).

Files with missing lines	Patch %	Lines
...tl-datadog/autoscaling/cluster/evict/evict_pods.go	78.94%	11 Missing and 5 partials ⚠️
...ubectl-datadog/autoscaling/cluster/evict/cordon.go	96.42%	1 Missing ⚠️

Additional details and impacted files

@@                          Coverage Diff                           @@
##           lenaic/CASCL-1386-evict-07-eks-mng    #3175      +/-   ##
======================================================================
+ Coverage                               44.11%   44.36%   +0.24%     
======================================================================
  Files                                     389      391       +2     
  Lines                                   31119    31283     +164     
======================================================================
+ Hits                                    13729    13878     +149     
- Misses                                  16498    16508      +10     
- Partials                                  892      897       +5     

Flag	Coverage Δ
unittests	44.36% <89.69%> (+0.24%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...d/kubectl-datadog/autoscaling/cluster/evict/asg.go	100.00% <100.00%> (+100.00%)	⬆️
...ubectl-datadog/autoscaling/cluster/evict/cordon.go	96.42% <96.42%> (ø)
...tl-datadog/autoscaling/cluster/evict/evict_pods.go	78.94% <78.94%> (ø)

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