Change core plugin format versioning scheme

Tristan Van Berkomrequested to merge
tristan/core-plugins-versioning into master

Use the global format-version for plugins which are maintained as a part of BuildStream, this is more simple to follow for users than separately revisioning plugins which are distributed with BuildStream anyway.

Also, this bumps the version for the new cmake plugin generator feature and adds an appropriate NEWS entry for the new feature.

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