[FEAT] in multidimensional histograms allow the binning to vary along an axis

[tmombaecher]

We are trying to implement a binning scheme (in 3D) where the binning varies along the axes (calibration of PID in kinematic bins at LHCb).
boost_histogram supports variable-sized bins via

import boost_histogram as bh
h = bh.Histogram(bh.axis.Variable(...), bh.axis.Variable(...), bh.axis.Variable(...))

but we would like to do it a bit more complex. A little example in a 2D grid between [0,1] for x and y:
binedges in x: [0,5.2]
binedges in y: [0,3.5] for the first bin in x and [0,5.5] for the second bin in x
(and then the extension of the same idea to 3D)