Placing in the air

**Current behavior:**

The `ar-placement` attribute has only 2 possible values: `"floor"` (default) and `"wall"`.

It's impossible to rotate and leave the model the way user want.

The 3D model is constantly turns bottom down.

The 3D model constantly falls to the floor.

This is only good for large objects like furniture. But it's not very convenient to view small 3D models like a mixer if they always fall on the floor. Users can still view small 3D models by twisting the arm (sometimes into an unnatural position), but as soon as user stops holding the trigger on the controller, the 3D model will fall to the floor.

**Expected behavior:**

The `ar-placement` attribute supports a new 3rd possible value: `"air"`.

The 3D model stays exactly in the position (location and rotation) in the air where the user left it.

E.g., kids can leave in the air some 3D model such as a human heart or a complex animated mechanism with gears *in their desired position*, and they can walk around it and look at it from all sides, including from the bottom.

**Related issues:**

In this case, the Hit Test API is optional, not required.

See also #4959 and #4960.