Add the extensibility attribute to allow specify custom steps

[MikDal002]

Description

Hello :)
During my work with Nuke, I've come across some places where increased extensibility would be a benefit. That's why I've developed an approach with custom steps. I would like to share with you my implementation.
As in shown in Usage Example, I think it is a good idea to add possibility to write own custom steps and address them by attributes. Considering the current architecture of Nuke, it gives a great extensibility without big amount of changes.

I've written the usage example from the top of my head because at this moment I do not have access to my code.

Usage Example

public class TestBuild : NukeBuild
{
    [AzureDevOpsDownloadSecureFileStep("securefile.zip")]
    public Target Test => _ => _
        .DependsOn(Compile)
        .Produces(TestResultDirectory / "*.trx")
        .Produces(TestResultDirectory / "*.xml")
        .Partition(2);
}
// User provided implementations
class AzureDevOpsDownloadSecureFileStepAttribute : AzurePipelineStepAttribute
{
    public AzureDevOpsDownloadSecureFileStepAttribute(string fileName)
    {
        FileName = fileName;
    }

    public string FileName { get; }

    public override AzurePipelinesStep Get()
    {
        return new AzureDevOpsDownloadSecureFileStep(FileName);
    }
}

class AzureDevOpsDownloadSecureFileStep : AzurePipelinesStep
{
    public AzureDevOpsDownloadSecureFileStep(string fileName)
    {
        FileName = fileName;
    }

    public string FileName { get; }
    public override void Write(CustomFileWriter writer)
    {
        writer.WriteLine("- task: DownloadSecureFile@1");
        using (writer.Indent())
        {
            writer.WriteLine("inputs:");
            using (writer.Indent())
            {
                writer.WriteLine($"secureFile: '{FileName}'");
            }
        }
    }
}

Alternative

Alternative is to write all YAML pipelines by hand, but I do not want that as in a team not everybody must be fluent with .yml files. That's why I like the idea to generate as much pipeline code as possible by Nuke.

Could you help with a pull-request?

Yes [#1407]