[ENH]: support rect with constrained_layout ("layout only to part of the figure") #22623

Problem

tight_layout supports a rect option ("layout axes so that they fit within this rect instead of filling the entire figure"). It could be nice if constrained_layout also supported that.

As an example use case, requested by @jklymak on gitter:

from pylab import *

fig = figure(layout="constrained")
axs = fig.subplots(1, 3)
for ax in axs:
    for i in range(3):
        ax.plot(rand(3), rand(3), label=f"dataset {i}")
axs[1].legend(ncol=3, loc="lower center", bbox_to_anchor=(0.5, 1.0))

fig = figure(layout="constrained")
axs = fig.subplots(1, 3)
for ax in axs:
    for i in range(3):
        ax.plot(rand(3), rand(3), label=f"dataset {i}")
fig.legend(handles=axs[0].lines, ncol=3, loc="upper center")

show()

This draws e.g. three different coordinates of three datasets, and tries to have a single one-line legend at the center top.

The first try gives

i.e. it would work if set_in_layout could be used to take the legend's y extent, but not its x extent (now that I think of it, that may be the better solution).
The second try gives

which is where I think a rect option would work to avoid the axes overlapping the legend (you'd have to tweak the value a bit manually, though, to specify the upper bound of the rect). (Perhaps not the nicest solution, but "could work when you just want to get a plot done".)

Proposed solution

No response