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

[Piedone]

Since https://github.com/Lombiq/Helpful-Libraries/pull/267 and https://github.com/Lombiq/UI-Testing-Toolbox/pull/377 we have dual-publishing to NuGet and Cloudsmith in HL and UITT. Only these projects, since these were needed for the preview versions of Orchard Core Commerce also published to Cloudsmith; we couldn't publish alpha versions of HL and UITT to NuGet for https://github.com/OrchardCMS/OrchardCore.Commerce/issues/442, since they depended on the preview version of OC, published to Cloudsmith.

However, these are specifically for *-preview-* tags, not substituting alpha releases. But, it's a good interim test.