Add Support for GitHub Actions

[eXpl0it3r]

Back when Jobbr was first started, GitHub Actions didn't exist and AppVeyor was the go-to Open Source CI for Windows/.NET applications. Since then GitHub Actions came to be and covers Windows, Linux as well as macOS.

Additionally, with moving Jobbr to .NET 6, it should essentially be possible to build it cross-platform.

As such, it makes sense to migrate the CI to GitHub Actions:

• [x] Jobbr.ComponentModel.ArtefactStorage
• [x] Jobbr.ComponentModel.Registration
• [x] Jobbr.ComponentModel.JobStorage
• [x] Jobbr.ComponentModel.Execution
• [x] Jobbr.ComponentModel.Management
• [x] Jobbr.Server
• [x] Jobbr.Storage.MsSql
• [x] Jobbr.ArtefactStorage.FileSystem
• [x] Jobbr.Execution.Forked
• [x] Jobbr.Runtime
• [x] Jobbr.Server.WebAPI
• [x] Jobbr.Dashboard
• [x] Demo

Examples that could serve as inspiration:

• https://github.com/eXpl0it3r/streamdeck-clockify/blob/master/.github/workflows/ci.yml
• https://github.com/SFML/SFML.Net/blob/master/.github/workflows/ci.yml

[eXpl0it3r]

Publishing part has been split off into #103 as I want to keep the publishing as-is for this release, but still add GitHub Action builds for this release, to ensure that everything can build on Linux as well.

[eXpl0it3r]

There were some interesting challenges and still room for improvement, but the whole Jobbr ecosystem (minus RavenDB and unfinished packages) have successfully built on Windows, macOS and Linux, incl. tests. 🎉