Check that MAR signing is done properly on the files available in the update responses

In legacy/trac#18405 (moved) we are adding a script to be used during the release process to check that the MAR files are properly signed. We could have an other one that is doing the same things on the files currently proposed as an update. This would allow someone to easily check (maybe as a cron job) that the updates currently available are the same as the ones in the sha256sums-unsigned-build files.

In tools/update-responses/check_update_responses_deployement we have a script that currently check that the update responses xml provides the expected version. I think I could extend it to also download the mar files it provides, unsign them and check that they match sha256sums-unsigned-build.txt.