golangci-lint: add stylecheck linter

[olivierlemasle]

Adding stylecheck linter, as it would have prevented duplicated import (#127).

This was prompted by this comment.

stylecheck adds the ST* checks from https://staticcheck.io/docs/checks (the SA* and S* checks were already enforced).

The duplicated imports fixed in #217 would have raised an error:

pkg/registry/custom_metrics/reststorage.go:28:2: ST1019: package "k8s.io/apiserver/pkg/endpoints/request" is being imported more than once (stylecheck)
	"k8s.io/apiserver/pkg/endpoints/request"
	^
pkg/registry/custom_metrics/reststorage.go:29:2: ST1019(related information): other import of "k8s.io/apiserver/pkg/endpoints/request" (stylecheck)
	genericapirequest "k8s.io/apiserver/pkg/endpoints/request"
	^
pkg/registry/external_metrics/reststorage.go:26:2: ST1019: package "k8s.io/apiserver/pkg/endpoints/request" is being imported more than once (stylecheck)
	"k8s.io/apiserver/pkg/endpoints/request"
	^
pkg/registry/external_metrics/reststorage.go:27:2: ST1019(related information): other import of "k8s.io/apiserver/pkg/endpoints/request" (stylecheck)
	genericapirequest "k8s.io/apiserver/pkg/endpoints/request"
	^

[k8s-ci-robot]

@olivierlemasle: This issue is currently awaiting triage.

If custom-metrics-apiserver contributors determine this is a relevant issue, they will accept it by applying the triage/accepted label and provide further guidance.

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[dgrisonnet]

Awesome! Thanks!

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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