Add support for strategic merge patch style

[112358fn]

The work done in #8490 to add structural OpenAPI schema of Tekton CRDs, enables the support for strategic merge patch.

According to the kustomize documentation

Strategic merge patches may require additional configuration via openapi field to work as expected with custom resources.

From the sig-api-machinery:

Strategic Merge Patch uses the go struct tag of the API objects to determine what lists should be merged and which ones should not. The metadata is available as struct tags on the API objects themselves and also available to clients as OpenAPI annotations.

This means that by including the extensions x-kubernetes-patch-strategy and x-kubernetes-patch-merge-key on the OpenAPI schema of the relevant lists we can enable strategic merge patch.