Filter labels/annotations by callbacks with arbitrary logic

[nolar]

What do these changes do?

Add filtering callbacks for individual labels/annotations, similar to the whole-object when= filtering callback.

Description

This is a little separate follow-up for #327:

• We have labels/annotations filters with values & markers for present/absent values.
• We have callback filters with arbitrary logic for the whole objects.

Why not make callback filters with arbitrary logic for specific labels/annotations?

Just a little feature, which combines the two already existing features:

import kopf


def check_marker(value, spec, **_):
    return value is not None and value == spec.get('field')


@kopf.on.create('zalando.org', 'v1', 'kopfexamples',  
                annotations={'some-marker': check_marker})
def kex_created(**_):
    pass

The value is either a str, or None for an absent label/annotation. The callback is called always.

The kwargs are the same as for the handler or a when= callback.

Issues/PRs

Issues: #98

Related: #258

Type of changes

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

Checklist

• The code addresses only the mentioned problem, and this problem only
• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• If you provide code modification, please add yourself to CONTRIBUTORS.txt

[zincr]

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