Create a dimensional packer

Problem/Motivation

The currently provided packer DefaultPacker creates a single shipment for an order. It does not select a package (box), account for dimensions, or pass a calculated dimension to shipping plugins to get accurate rates based on dimensions.
This is a hard problem to solve. But even a non perfect dimensional packer would be a huge improvement for sites that use shipping services that charge rates based on package dimensions in addition to weight.

Proposed resolution

Create a dimensional packer. At it's most basic, just adding up the dimensions of all the order items and splitting shipments if they exceed the dimensions of available packages (boxes) configured would be a good first step. It wouldn't play tetris to get the most optimal size but at least it would have a measurement.

Remaining tasks

User interface changes

Is there a way to choose which packer a shipping method uses? How will users decide which packer to use?

API changes

Data model changes