Label sub-templates recursively

[nolar]

Issue : #107, indirectly #13

Now, it is a bit easier to label both the created object (e.g. jobs) and its sub-templates (e.g. pods in jobs):

kopf.label(job, {'parent-name': name}, nested=['spec.template'])

Also, few tests added on the utility functions used internally, just to be sure they work as expected.

The auto-guessing on which sub-fields are templates, which are not (as suggested in #107) is skipped for now — that might require enumerating the kinds&fields from the docs.

Types of Changes

• New feature (non-breaking change which adds functionality)

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[zincr]

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Details on how to resolve are provided below

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Approvals

All proposed changes must be reviewed by project maintainers before they can be merged

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• ✅ Approved by PR author @nolar
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[samurang87]

must be the first time I see recursion in "real" code!