enhancement: improve user access to private registries

[abangser]

trafficstars

Right now, people need to provide node level permissions to gain access to any SKE private docker images (see: https://github.com/syntasso/kratix-docs/issues/41).

This is less than ideal as it is less secure than using imagePullSecrets which is built into the Kubernetes workflow.

The quick fix here is to expose imagePullSecrets as a field in the Workflow Pipeline kind., but we may want to think about the user experience of needing this extra field for all SKE images vs providing a convention that can reduce overhead.

If we go conventions, we may want to somehow let the user configure that they are SKE customers, and provide us the token. From there we can take ownership of creating the secrets in each namespace and reading them by default in each workflow. We would still likely want to provide the field explicitly as people may need this field for their own images and/or want to buck our convention for some reason.