Checksum for published packages

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## Feature request

**Which Nextcloud Version are you currently using:** (see administration page)
4.0.0

**Is your feature request related to a problem? Please describe.**
A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]

When building a package for a distro and upon receiving a contribution it is quite important to know the tarball is authentic. A checksum posted together with the tarball would be most helpful.

**Describe the solution you'd like**
A clear and concise description of what you want to happen.
B
a checksum file together with each published package to verify authenticity., e.g. sha256sum

**Describe alternatives you've considered**
A clear and concise description of any alternative solutions or features you've considered.

**Additional context**
Add any other context or screenshots about the feature request here.