Make K8s secrets watchable (#680)

[mhartmann-jaconi]

Extracts common watch functionality for secrets and config-maps into abstract base classes and implement those for the dedicated sub types, so that watch config looks the same for both types and has the same base implementation.

[CLAassistant]

All committers have signed the CLA.

[graemerocher]

@n0tl3ss please test this feature in production k8s deployment and resolve the failing tests. Thanks.

[n0tl3ss]

@graemerocher I have tested code it works and looks ok to me :) nice work @mhartmann-jaconi

[graemerocher]

Thanks for the contribution!