EF8: Explore support for JSON columns

[bkoelman]

Is your feature request related to a problem? Please describe.

EF7 added support for JSON columns, enabling to map the JSON structure inside a database column as entities and update their contents.

Describe the solution you'd like

We should explore how well this works with PostgreSQL and possibly create a test/sample demonstrating how to use it.

Related to https://gitter.im/json-api-dotnet-core/Lobby?at=635fc354dcbf067fdcbd5aab.

[bkoelman]

In EF7, this only works on SQL Server. It's on the roadmap of EF8 to support other databases, address existing limitations and improve performance.

[bkoelman]

Related to #1271.

[bkoelman]

Closing in favor of #1439.