Never map projectable properties

[koenbeuk]

Fixes #109

PR Classification

New feature to enhance Entity Framework Core with a custom convention.

PR Summary

Introduces a convention to automatically ignore properties marked with ProjectableAttribute in Entity Framework Core.

• ProjectablePropertiesNotMappedConvention.cs: Added a class to ignore properties marked with ProjectableAttribute.
• ProjectablePropertiesNotMappedConventionPlugin.cs: Added a plugin to include the new convention in the convention set.
• ProjectionOptionsExtension.cs: Updated to register the new convention plugin.
• EnumerableProjectableTests.cs: Added tests to verify the new convention.