Ensure that YAML marshalling matches the JSON marshalling

[damien-talos]

I noticed that some types used MarshalYAML as the source of truth for MarshalJSON, but others didn't.
Some of those were out of sync, especially with regards to the serialization of extensions.

This PR updates all the MarshalJSON functions to call the MarshalYAML one, with the result that both JSON and YAML marshalling should result in the same value being written.

[fenollp]

Hi! Thanks for submitting this. Should we be thinking of a regression test?
Please rebase your PR, otherwise LGTM :)

[damien-talos]

@fenollp Certainly, I'll rebase my PR.
I'm happy to add tests, though I'm not sure what that would look like?
Perhaps somehow generating tests that serialize various objects to both JSON and YAML, than de-serialize them, and compare them to ensure they are equivalent? Is there existing code to generate tests like that, or would they have to be hand-coded?

[fenollp]

Such tests already exist, I was just wondering if you'd thought of something new. No worries, let's just rebase :)

[damien-talos]

I did a rebase here, and fixed a few issues, but I've now got tests that are failing because the properties are serialized in a different order.
How important is the order of the properties in the serialized output? I could update the snapshots to reflect the new order (which seems to be alphabetical), but I'm not sure if that's desired or not?