Add helper functions to create AxisAlignedBox objects from other shapes

[scpeters]

Desired behavior

Add helper functions that create an AxisAlignedBox object when passed a shape object like Box, Capsule, Cylinder, etc.

Alternatives considered

Implementation suggestion

Add static helper functions to a separate header file (such as include/gz/math/AxisAlignedBoxHelpers.hh) with the following API:

static gz::math::AxisAlignedBox ConvertToAxisAlignedBox(const gz::math::Box &);
static gz::math::AxisAlignedBox ConvertToAxisAlignedBox(const gz::math::Sphere &);
static gz::math::AxisAlignedBox ConvertToAxisAlignedBox(const gz::math::Capsule &);
etc.

Additional context

This was motivated by the feature proposed by @gabrielfpacheco in gazebosim/sdformat#1547.