Drag and drop, ability to hold element while drag

#### Please describe the feature you would like to request.
Ability to pass boolean value for hold element while moving. it is something like `dragStartThreshold`, but during dragging. (it will block to change transform3d values till we change hold value e.g `cdkDragHold = false`)

#### What is the use-case or motivation for this proposal?
for example, if a user drags an element past near the center of the page, we need to magnetize the element with center for 20px area, then we pass new input e.g. `cdkDragHold = true` and by that we want to hold dragging element till we change this value. I think this might be useful for another many cases, bcs it's quite hard for user to moving element exactly pixel in pixel in some cases, but we can help to do this programmatically.
#### Is there anything else we should know?
It a bit similar to `dragStartThreshold` from `DragRefConfig`.

まず、既存の DragRefConfig と dragStartThreshold の動作を読み、次にドラッグ中に cdkDrag が transform3d の値をどのように更新するかを追跡します。hold 入力が要求された磁化された 20px の中央領域とどのように相互作用するかを定義し、値が変化したときに移動が一時停止して再開することを示すテストカバレッジを追加します。

索引モデルが issue の本文から書いたものです。