[feat]: add method for validating Jenkinsfile

[eckdanny]

Add an RPC style method on jenkins for validating a Jenkinsfile (see pipeline docs)

usage something like:

jenkins.validate(Jenkinsfile: string, (err, data) => {
  if (err) throw err;
  something(data);
})

The Jenkins remote API is pretty much:

POST /pipeline-model-converter/validate
     Headers.ContentType = multipart/form-data
     {
       jenkinsfile: <stringified-jenkinsfile-contents>
     }