roachtest: add EntityFramework Core nightly tests

[rafiss]

Instructions from the EFCore team:

• Clone the Npgsql EF Core provider
• Edit the test/EFCore.PG/config.json to point at the Cockroach host/port/database/username/password (the connection string format is documented here).
• Run the tests on the command line by executing "dotnet test" under test/EFCore.PG (after installing dotnet SDK 3.1)
• You can also use various IDEs to open the solution and run the tests inside, e.g. Visual Studio, VS Code, or my personal favorite, Jetbrains Rider.

cc @roji

Jira issue: CRDB-4207

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