feat(common): add ExperimentalGCPPlanTerm to RecommendationParams

[cristim]

Adds only the RecommendationParams.ExperimentalGCPPlanTerm field (default false, EXPERIMENTAL doc comment). No behavior change: nothing reads it yet.

Split from #401 so providers/gcp can bump its pkg pin before consuming the field (standalone module CI builds with GOWORK=off). Refs #291.

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Reviewing files that changed from the base of the PR and between a4fc226 and 93b86c3.

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• pkg/common/types.go

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[cristim]

Gate verdict at head 93b86c33893dc52a760e7c589b905bc54de3e246 (base main a4fc226, up to date): PASS.

• Diff is only pkg/common/types.go +5: the ExperimentalGCPPlanTerm bool field and a doc comment saying it is EXPERIMENTAL and that the default false keeps Term ("1yr"). Nothing reads it, so behavior is unchanged.
• No unkeyed RecommendationParams{...} literals and no JSON marshalling of the params in the repo (git grep), so adding the field breaks no caller.
• Local (GOTOOLCHAIN=go1.26.9, git archive at head): pkg GOWORK=off go vet + go test -race -short exit 0. With GOWORK=off, ci_cd_sanity_tests, providers/aws, providers/azure and providers/gcp all build against their current pins (exit 0). With replace pkg => ./pkg (simulating the future pin bump), each builds, vets and runs go test -short with exit 0.
• CI: all checks green, mergeStateStatus CLEAN. Labels present.