Allow users to publish only a subset of changesets

[mrnugget]

With the switch to the new workflow publishing changesets is an all-or-nothing operation.

Right now, to publish the changesets in a campaign the user has to switch the published: false in the campaign spec

# ...
changesetTemplate:
  # ...
  published: false

from false to true.

We should allow users to publish only a subset of changesets.

---

More generally: we should allow users to set all of the attributes in the changesetTemplate (commit message, title, body, ...) for only a subset of repositories.

[LawnGnome]

Here is a draft RFC containing some (but probably not all) of the available options.

[davejrt]

Dear all,

This is your release captain speaking. 🚂🚂🚂

Branch cut for the 3.20 release is scheduled for tomorrow.

Is this issue / PR going to make it in time? Please change the milestone accordingly.
When in doubt, reach out!

Thank you