Implement an infrastructure for auto discovery of evaluators and validators.

[fiseni]

Added the ability to automatically discover the partial evaluators and validators. The users, once they add implementations for IEvaluator, IMemoryEvaluator, or IValidator, no longer need to add/register them manually. They will be discovered automatically and added to the pipeline.

I wanted to expose this as an optional feature. I added targets as part of the NuGet package, and the feature can be enabled by setting the following property in your .csproj file

<PropertyGroup>
  <SpecAutoDiscovery>enable</SpecAutoDiscovery>
</PropertyGroup>

Additionally, a discovery attribute can be applied to each evaluator/validator individually. You can control the order and also have the ability to disable a specific evaluator/validator from discovery. The default order is int.MaxValue and is enabled by default. If no attribute is applied, the default values will be assumed.

[EvaluatorDiscovery(Order = 1000, Enable = true)]
public class TestEvaluator : IEvaluator
{
    public IQueryable<T> Evaluate<T>(IQueryable<T> source, Specification<T> specification) where T : class
    {
        // Your implementation here
        return source;
    }
}

[EvaluatorDiscovery(Order = 1000, Enable = true)]
public class TestMemoryEvaluator : IMemoryEvaluator
{
    public IEnumerable<T> Evaluate<T>(IEnumerable<T> source, Specification<T> specification)
    {
        // Your implementation here
        return source;
    }
}

[ValidatorDiscovery(Order = 1000, Enable = true)]
public class TestValidator : IValidator
{
    public bool IsValid<T>(T entity, Specification<T> specification)
    {
        // Your implementation here
        return true;
    }
}