Allow to add prefix/suffix to release notes

Loic Nicollerequested to merge
allow_prefix_suffix_release_notes into main

For some repo we may want to add some info in the release note, before or after the list of merged MR.

eg for diskimage-builder we want to add the list of the build images at the end of the release notes.

This could be achieve by overriding .release-notes-suffix: with some script to append release-notes.md

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