Restructure module to allow importing as Nuget package in another binary module

[PalmEmanuel]

Needs some fun restructuring and added complexity of module projects. Suggestion for new structure:

• AzAuth.Core - Contains core auth logic and wraps MSAL packages. Will be loaded into custom ALC for isolation. Same as now.
• AzAuth.Abstractions - New project which will expose an abstract AzAuthGetTokenCmdlet which can be implemented by a PowerShell module through inheritance. This will be published as a Nuget package.
• AzAuth.PS - Implements AzAuth.Abstractions and its abstract AzAuthGetTokenCmdlet, as well as the cache cmdlets like today. Will instead use a new project to manage loading assemblies, since the AzAuth.Abstractions with the cmdlet must be accessible through by PowerShell (loaded into default ALC)
• AzAuth.Loader - Managed specific loading of assemblies, uses [ModuleInitializer] to run code when AzAuth.Loader assembly is loaded (because AzAuth may itself be loaded into a custom ALC so it can't load the AzAuth.Abstractions into the current ALC). AzAuth.Abstractions is loaded into default ALC, AzAuth.Core is loaded into custom ALC.

May need tweaking but should work.