Argument for additional metrics in model_performance()

I'd like to be able to request additional performance metrics in model_performance(), such as AICC or confidence intervals for R2 or sigma. It would be nice if, in addition to the metrics argument, there were an additional_metrics argument that could take a named list of additional functions to run, each of which should return a vector a results that would be added as additional columns to the performance data frame.

For example:

m <- lm(formula = mpg ~ wt + qsec + am, data = mtcars)
performance(m, metrics = "common", additional_metrics = list(`R2 CI` = confint_R2, AICC = performance_aicc))

Or perhaps it would be cleaner to just have the one metrics argument and allow it to be either the current string vector or a named list of functions:

m <- lm(formula = mpg ~ wt + qsec + am, data = mtcars)
performance(m, metrics = "list(R2 = r2, `R2 CI` = confint_R2, AICC = performance_aicc, Sigma = get_sigma))

Start at the R package's model_performance() entry point and inspect how the current metrics argument selects performance measures. Compare the additional_metrics named-list proposal with the alternative of accepting named functions in metrics, then use the supplied lm example to define how custom results should become columns in the performance data frame.