Make the ShortenerTool.Core a completed package

[FBoucher]

Discussed in https://github.com/microsoft/AzUrlShortener/discussions/476

^{Originally posted by FBoucher March 10, 2023} The idea is to move all the logic into ShortenerTool.Core, without any security check. This way we could create multiple wrapper that implement the security we need (or none) depending on the context and usage. It could be a NuGet package and be easy to integrate in many projects. We could finally work on the CLI we've been dreaming about for so long.

Any thoughts? Feedback? Warning? Push-back?