feat: support for diffing presence containers

[JonasKs]

Closes #936.

The implementation ended up being somewhat as described in #936, where we indicate that presence container has a nil value in the findSetLeaves function.
In addition to this, we added a presence container interface, which is used in the marshalStructOrOrderedList to isolate the presence container which might have values, to act like an empty struct.

The result is that we do not break any current tests (even without the diff option), but we decided to add a diff option, with the reasoning described in #936:

Adding this as a non-optional, paired with ygot.BuildEmptyTree would also probably be breaking, as it would set all these presence containers?

We've confirmed this works in our setup. We're able to successfully configure all presence containers, even nested ones.

Co-authored-by: Terje Lafton terje@lafton.io

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[JonasKs]

Hello @wenovus and @robshakir - it took some time, but we finally managed to crack the code. Please review. 😊

[coveralls]

coverage: 88.766% (-0.04%) from 88.807%
when pulling bcaa4dc on JonasKs:support-presence-containers
into 100bd44 on openconfig:master.

[JonasKs]

Also tagging #645 author, @hansthienpondt.

[JonasKs]

Hi @robshakir , friendly bump in case you missed this 😊

[robshakir]

I did miss it -- apologies, let me review ASAP.

[JonasKs]

I did miss it -- apologies, let me review ASAP.

No worries, @robshakir! We’ve been using the fork in production and it’s worked like a charm.

[JonasKs]

@robshakir, would love to get this looked at. 😊

[robshakir]

@JonasKs holy-moly! apologies! this one passed me by. reviewing now. 🔎

[JonasKs]

Thanks so much, @robshakir! I’m out of office for ~2 weeks, but I’ll address all comments when I’m back!

[JonasKs]

Hey @robshakir, sorry for never following up here. I no longer work at Intility, but I got a call from a good old colleague @luancvt, who still needs this merged upstream. I've addressed the comments here, and would appreciate if you could do another review?

I've given @luancvt write access to my fork, so he can follow up any feedback you might have.

Seems like pipelines fail in master nightly checks too, so I'll leave those.