Allow to set schema for entity if they use RDBMS

[thangchung]

• Define attribute to allow developer to set schema for entity
• Decorate on each entity
• Modify code at https://github.com/cloudnative-netcore/netcorekit/blob/master/src/NetCoreKit.Infrastructure.EfCore/Db/AppDbContext.cs#L121 as the code in https://github.com/thangchung/blog-core/blob/master/src/Framework/BlogCore.Infrastructure.EfCore/ModelBuilderExtensions.cs#L17