Split DAOs into own NuGet packages

[Arithmomaniac]

Moving each provider into its own Nuget package has a few advantages:

• Easier to navigate
• Enforces separation of concerns - cross-DAO stuff cannot reference any Relativity-specific DLLs
• Allows different implementations to set different Relativity requirements (for example, an ExportAPI provider would set a minimum version of 9.7; SqlDao doesn't need ObjectManager
• Allows for targeting platforms that Relativity doesn't get (such as a pure REST-based implementation that world work on .NET core)

We should have:

• A core Gravity.Core package with common logic and models
• Separate packages for each provider
• A Gravity metapackage that will install all of the other packages (providing backwards compatibility for referencers)

[KalinaTSD]

Sounds helpful for the other developers. They will know what we do if we separates all packages in such way.