feat(backend): ARTIFACT_COPY_STEP_TEMPLATE - custom template for copy step

[rafalbigaj]

Extend artifacts copy step configuration with the container template.

Which issue is resolved by this Pull Request:
Resolves #333

Description of your changes:
The new ARTIFACT_COPY_STEP_TEMPLATE configuration option is introduced that allow to provide a customisable copy step container template. It is extremely useful in case of custom steps that require access to specific secrets or env variables.

Environment tested:

• Python Version (use python --version):
• Tekton Version (use tkn version):
• Kubernetes Version (use kubectl version):
• OS (e.g. from /etc/os-release):

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[kubeflow-bot]

This change is 

[Tomcli]

Thanks @rafalbigaj, can you add a small section to show what the ARTIFACT_COPY_STEP_TEMPLATE should look like? You can put it under https://github.com/kubeflow/kfp-tekton/blob/master/kfp-admin-guide.md#customize-artifact-image-to-do-your-own-post-processing

[Tomcli]

@rafalbigaj since this feature doesn't break any usability on our side, I can merge it in for now so that we can include it in our 0.4 release. But it will be nice if you can also document how to use it.

[Tomcli]

/lgtm
/approve

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: rafalbigaj, Tomcli

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