Support iris.plot.fill_betweenx and iris.quickplot.fill_betweenx

[SimonPeatman]

✨ Feature Request

#4647 implemented an iris.plot wrapper for matplotlib's fill_between function but not fill_betweenx. Was this an oversight or is there a reason why fill_betweenx is trickier to implement?

[bjlittle]

@SciTools/peloton Hey @SimonPeatman, thanks for raising this issue.

It just implemented fill_between on request from a users, but it would make total sense to also support fill_betweenx ... fancy taking a punt at it?