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

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

ACR: TWDAPI-274

[Verification] Code tested in Chrome DevTools with below snippets works
fine

var pipeline_def = "videotestsrc is-live=true"
                   + " ! videoconvert"
                   + " ! tensor_converter"
                   + " ! output-selector name=outs outs.src_0"
                   + " ! tensor_sink name=sink0 async=false outs.src_1"
                   + " ! tensor_sink name=sink1 async=false"

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

pipeline.getSwitch('outs')
// Switch {name: "outs", type: "OUTPUT_SELECTOR", _pipeline_id: 3}

pipeline.getSwitch('non existent switch')
//  VM31:1 Uncaught WebAPIException {code: 0, name: "InvalidValuesError"

Change-Id: Ia7d6838b5e49072c35f0b796151c436d4a3b60e1
Signed-off-by: Pawel Wasowski <p.wasowski2@samsung.com>