Add sha256 field in package file

[valentinmanea]

Added the sha256 field in PackageFile class

Fixes: #1142

[jmini]

Please add a value in the example file package-files.json. This way we are sure the serialisation/deserialisation code works (tested in the TestGitLabApiBeans unit test)

[jmini]

I have rebased the source branch of this PR (6.x_add_sha256_field_in_package_file in the valentinmanea fork) on top of main of this repo:

git rebase --onto origin/main origin/6.x
git push valentinmanea HEAD:6.x_add_sha256_field_in_package_file --force

Be aware that your local branch 6.x_add_sha256_field_in_package_file has diverged with the one on the GitHub.

If you need to do one more changes on this branch:
I recommend to use git fetch and git reset --hard origin/6.x_add_sha256_field_in_package_file (assuming your local origin is pointing to the valentinmanea fork on GitHub) before doing any local modifications instead of git pull.

I have changed the target branch of this PR to be main

[valentinmanea]

@jmini Can you merge this ?

[jmini]

I have been fighting CI, because of the flakiness of some integrations tests. But now it is good.

[jmini]

I have merged your change to main and 6.x. 🎉

Thank you very much for this contribution.