[ML][pipeline] Implement Pipeline::getValve()

[ML][pipeline] Implement Pipeline::getValve()

ACR: TWDAPI-274

[Verification] Tested in Chrome DevTools with the below snippets, works
fine.

var pipeline_def = "videotestsrc is-live=true ! videoconvert ! videoscale"
                   + " ! video/x-raw,format=RGBx,width=16,height=16,framerate=10/1"
                   + " ! tensor_converter ! valve name=valve1 ! fakesink";

var pipeline = tizen.ml.pipeline.createPipeline(pipeline_def,
                                      function(state) {console.log(state);});

pipeline.getValve('valve1')
// alve {name: "valve1", _pipeline_id: 1}

pipeline.getValve('valve_XXX')
// WebAPIException {code: 0, name: "InvalidValuesError", ...

pipeline.getValve()
// WebAPIException {code: 0, name: "InvalidValuesError", ...

Change-Id: Id11414f80c42a9879532dfc1c651f766c4f12834
Signed-off-by: Pawel Wasowski <p.wasowski2@samsung.com>