EF Core: Add provider extension points for DBContect registration and database configuration.

[nikolajlauridsen]

This PR adds two new interfaces: IDbContextServiceRegistrar for registering services for every registered DbContext and IDatabaseConfigurator for configuring DbContexts (calling UseSqlite etc). This allows us to move the distributed locking mechanism out of Infrastructure and drop the reference to Microsoft.EntityFrameworkCore.SqlServer and Microsoft.EntityFrameworkCore.Sqlite, and also means there's no need for Umbraco.Cms.Persistence.EFCore cause everyhing that's not in infrastructure can be done from the respective packages.

Testing

Ensure that tests pass and backoffice works as intended