Publish alpha packages to Cloudsmith (OSOE-649)

[Piedone]

Currently, we publish all issue-specific alpha versions of our projects' NuGet packages to nuget.org. Not to pollute NuGet, let’s publish them to Cloudsmith instead.

• This can be done with a different NuGet publish GHA workflow for non-release (e.g. alpha) version tags. OCC does this with preview tags, see here.
• Add the necessary NuGet.config to OSOCE to configure the Cloudsmith feed. For projects with a dependency on another project of ours (like Helpful Extensions) this is needed too. We can just auto-add a centrally managed such file to every project, like we do with the Lombiq logo.
• Add a shield for the Cloudsmith package to the Readme of all projects too.
• Update our Open-source template too.

Jira issue