Customize content export

[gforcada]

Part of #86

This changes are actually a no-op for regular users of plone-exporter and plone-importer cli tools.

What this change allows is to customize on you own add-ons or policy products what should be exported when calling plone-exporter.

[mister-roboto]

@gforcada thanks for creating this Pull Request and helping to improve Plone!

TL;DR: Finish pushing changes, pass all other checks, then paste a comment:

@jenkins-plone-org please run jobs

To ensure that these changes do not break other parts of Plone, the Plone test suite matrix needs to pass, but it takes 30-60 min. Other CI checks are usually much faster and the Plone Jenkins resources are limited, so when done pushing changes and all other checks pass either start all Jenkins PR jobs yourself, or simply add the comment above in this PR to start all the jobs automatically.

Happy hacking!

[gforcada]

@jenkins-plone-org please run jobs

[davisagli]

The idea here looks ok but:

• needs a test
• needs docs
• what happens if there are other export steps that refer to content which was not included by this adapter?

[gforcada]

The idea here looks ok but:

* needs a test

✅ added

* needs docs

✅ added, see plone/documentation#2101

* what happens if there are other export steps that refer to content which was not included by this adapter?

I added a warning on the docs, that as soon as you customize the export, it might be that you need to further fine tune the other exporters.

Given that this is not the default and you have to create the adapter and override it etc. I would expect that if you know how to do that, you can deal with a maybe(?) broken export, but that's what you coded, so it's up to you to deal with it.

[davisagli]

Given that this is not the default and you have to create the adapter and override it etc. I would expect that if you know how to do that, you can deal with a maybe(?) broken export, but that's what you coded, so it's up to you to deal with it.

@gforcada Have you tried to use this already with your own IObjectsExporter adapter override? I guess I'm okay being a little hand-wavy about this, but I'd like to make sure that it at least serves your needs before we lock in the interface.