Incremental checkpoint for Track2 readiness.

[digimaun]

• SDK candidate now targeting netstandard2.0
  • This means we have to downgrade our C# syntax from 8 to 7.
  • Also means System.Text.Json is no longer built-in we need an explicit dependency.
• Overhaul of ctors
  • Simplest ctor will automatically point to the prod model repo.
• dmr-client will initialize ResolverClient with DependencyResolutionOption.FromExpanded
• Removal of factory initializers
• ResolverClientSettings renamed to ResolverClientOptions to better fit the nomenclature of Track2.
• One overload of ResolveAsync() (the one with params) had to be removed to prepare for plumbing of cancellation token where parameter order matters.
• Removes unnecessary IsValidDtmi from RepositorHandler, as the ResolverClient can call DtmiConventions directly.
• Add test for inline extends and variant
• Update tests