RHOAIENG-30439: tekton: factor out the common multiarch-pull-request-pipeline into its own file (#1501)

This is according to advice from Andrej, following the implementation in
https://github.com/konflux-ci/olm-operator-konflux-sample/tree/7b9d2db1312a6d542c6711ff87810dcbf5ed07da/.tekton

Author: jiridanek