feat: add get screens endpoint

[KazuCocoa]

e.g.

With external display via iPad.

{
  "value" : [
    {
      "scale" : 2,
      "traits" : 1,
      "isMain" : true,
      "bounds" : {
        "y" : 0,
        "x" : 0,
        "width" : 1640,
        "height" : 2360
      },
      "displayId" : 1
    },
    {
      "scale" : 2,
      "traits" : 4,
      "isMain" : false,
      "bounds" : {
        "y" : 0,
        "x" : 0,
        "width" : 6016,
        "height" : 3384
      },
      "displayId" : 2
    }
  ],
  "sessionId" : null
}

---

/wda/screen shows current 'main' screen while this endpoint shows available screens.

This non-main display does not function right now, but could extend some features at least such as screenshot, element handling etc. For example, XCUIElement and XCElementSnapshot have displayID

I won't add this endpoint call from xcuitest driver only with this change.

[mykola-mokhnach]

should it also be marked as standalone?