plot_metric_boot: Plot the bootstrapped metric distribution from a cutpointr object
Description
Given a cutpointr object this function plots the bootstrapped metric distribution,
i.e. the distribution of out-of-bag metric values.
The metric depends on the function that was supplied to metric in the
call to cutpointr.
The cutpointr function has to be run with boot_runs` > 0 to enable bootstrapping.