[FEATURE] Recovery plot error bars for asymmetric posteriors

[sjedhoff]

Currently, in the recovery diagnostic plot, the uncertainty of the posteriors (shown via error bars) is computed using the MAD through the uncertainty_agg argument. This can be misleading, particularly when dealing with asymmetric posterior distributions.

A more informative approach would be to allow error bars to represent credible intervals (e.g., 90% or 95%). This would capture asymmetry in the posteriors more accurately and provide a clearer picture of the underlying uncertainty.

[vpratz]

Closed by #573