Ingest Pipeline processors if condition support

[octocat-mona]

NEST/Elasticsearch.Net version:
6.3.1

Elasticsearch version:
6.5.0

Description of the problem including expected versus actual behavior:
All processors have an if condition option, but this is not exposed in for example the AppendProcessorDescriptor (or any other IProcessor).

Steps to reproduce:

1. Create pipeline:

await _service.PutPipelineAsync("my-pipeline", descriptor => descriptor
    .Description("My Pipeline")
    .Processors(processorsDescriptor => processorsDescriptor
        .Append<object>(appendDescriptor => appendDescriptor
            .Field("my_field")
            .Value("the_value")
        )
    )
);

2. Note there's no option for .If() like there is for .OnFailure()

Describe the feature:
Expose a method to configure an If condition just like OnFailure

[russcam]

Opened #3528