feat: add possibility to override buildpack's `build` step

[kseniyashaydurova]

https://saritasa.atlassian.net/browse/CDAU-2

Related PR: https://github.com/saritasa-nest/cpca-kubernetes-azure-v2/pull/1 (That's how it is used - https://github.com/saritasa-nest/cpca-kubernetes-azure-v2/blob/feature/add-prod/prod/argocd/apps/values.yaml#L1162)

Add new overrideBuildStep parameter to be able to override Buildpack's build steps. It may be needed to perform different docker auth, i.e. like in the example for Azure.

  buildpacks:
    enabled: true
    generate:
      buildpackFrontendBuildPipelineNew:
        name: buildpack-frontend-build-pipeline-new
        enabled: false
        buildTaskName: buildpack-frontend-new

        overrideBuildStep:   <---------------------------------------- new param
          name: build
          image: node:16
          imagePullPolicy: IfNotPresent
          workingDir: $(resources.inputs.app.path)
          script: |
            #!/bin/bash
            az login --identity --username <managed-indentity>
            az acr login --name <container-registry>
            /cnb/lifecycle/creator \
              -app=$(params.source_subpath) \
              -project-metadata=project.toml \
              -cache-dir=/cache \
              -layers=/layers \
              -platform=$(workspaces.source.path)/$(params.platform_dir) \
              -report=/layers/report.toml \
              -cache-image=$(params.cache_image) \
              -uid=$(params.user_id) \
              -gid=$(params.group_id) \
              -process-type=$(params.process_type) \
              -skip-restore=$(params.skip_restore) \
              -previous-image=$(resources.outputs.image.url) \
              -run-image=$(params.run_image) \
              $(resources.outputs.image.url)

        buildTaskSteps:
          - name: hello1
            image: node:16
            imagePullPolicy: IfNotPresent
            workingDir: $(resources.inputs.app.path)
            script: |
              #!/bin/bash
              echo "hello world1"
          - name: hello2
            image: node:16
            imagePullPolicy: IfNotPresent
            workingDir: $(resources.inputs.app.path)
            script: |
              #!/bin/bash
              echo "hello world2"