✨ Move mergeMap to util

[LuBingtan]

What this PR does / why we need it:
Move mergeMap in a shared util
Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #8276

[k8s-ci-robot]

Hi @LuBingtan. Thanks for your PR.

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[killianmuldoon]

/ok-to-test

[fabriziopandini]

/lgtm

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 2e742d582233295bdf1c0a832e0be940a1356fab

[killianmuldoon]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: killianmuldoon

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[killianmuldoon]

/retest

[johannesfrey]

/area util