Add UID to the metadata for summaries, for example related items.

[mauritsvanrees]

Fixes https://github.com/plone/plone.restapi/issues/1585

TODO: fix tests.

[mister-roboto]

@mauritsvanrees 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!

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[davisagli]

Note: This was added for serialized RelationValues in https://github.com/plone/plone.restapi/pull/1605. I think this change is probably still desirable, in order to include the UID in other places that use the summary serializer. But the tests need to be fixed.