Use UUID as entity ID

After talking to @rnsrk abou this in Burgas, spent some more time on this.

Fixed the issue mentioned in #9. Also added some inline comments about the changes and finally added some test coverage. I pushed those to individual commits in case there are any objections to any of those.

Note that I thought a bit about how to avoid all this array madness (i.e. array_values(array_unique(array_filter(...))) and friends) and I think the solution would be to make DefaultTableMapping provide an addFields() method, which would internally check for duplicate fields. Will open a follow-up about that, so we can avoid that discussion here (even though it does make the situation (inherently) slightly worse).

Not exactly why this was tagged "Needs change record" or what the change record should be. I mean we could literally write a change notice with "Content entities may now use the same field for the ID and UUID keys" if people think that's useful. (I don't, in particular, but it also doesn't hurt, so I wouldn't mind it.)

Not sure why the testbot doesn't run for the updated commits, but I opened 📌 Replace DefaultTableMapping::setFieldNames() with ::add*() and ::remove*() Active in the meantime.

...ahh maybe because the MR is targeting 10.1.x. @bradjones1 do you have access to change the target branch? Or should we create a new MR? Not sure...

MR target changed.

Needed a rebase and now GitLab CI is running.

Added a basic CR. Tests green now.

Awesome, thanks. You dropped my commits from the MR, though, I'm guessing that was accidental? 🤔

That is so strange, I think something weird happened with changing the merge target. I thought it was strange I didn't need to do a rebase --onto... and clearly that would be necessary. I've fixed this. Thanks for catching it.

Great, thanks! 👍