architecture improvements

[adamfoneil]

ModelSync has worked well for me personally, but there are some dependencies and assumptions it makes that hurt its marketability:

• Data Sources should support for .NET Standard in addition to Core #36
• Db-related metadata (constraints, keys) must be expressed with AO.Models attributes, which is too restrictive for a general-purpose tool. There should be a new "db dialect" abstraction layer that allows different db-related attribute conventions. The current AssemblyModelBuilder is where the AO.Models dependencies are. See issue 29 abstract conventions.
• there is limited support for db-to-db merges (library issue 16). That issue needs to be closed. The main, incomplete, work on this is here

[adamfoneil]

@usmanasfandiCheetah please target the first bullet above. I like what you're doing here, but seems like it needs support for CustomAttributeContext I think was the issue so far?

[adamfoneil]

Closing this issue to move it over to ModelSync back end