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`.

Commencez par lire le comportement existant de DragRefConfig et de dragStartThreshold, puis suivez la manière dont cdkDrag met à jour les valeurs de transform3d pendant le glissement. Définissez comment une entrée de maintien interagirait avec la zone centrale magnétisée de 20px demandée, et ajoutez une couverture montrant que le mouvement se met en pause et reprend lorsque la valeur change.

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