Correctly map metadata

[JakobFels]

Fixes #1188

This change will correctly write annotations and labels to a v3 manifest. When using v3 push labels and annotations were not correctly applied/serialized.

This change consists of 2 commits. The second one ensures backward compatibility with existing code setting labels or annotations, as to not require a new major version.

[linux-foundation-easycla[bot]]

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[anthonydahanne]

@JakobFels thank you for the contribution! We are working through PRs at the moment.

Could you please add integration test support to cover the new feature?

As an example, annotations checks can be seen in the integration-test module in places such as here