[math] Support AD for TMath::LnGamma using functions from GSL

This is to avoid any num-diff fallback in RooFit, which results in
annoying warnings for the user.

A new function `ROOT::Math::digamma` is added to the public interface,
which wraps `gsl_sf_psi`. The digamma function is the derivative of
`lgamma`, so it is used in `CladDerivator.h` to define the derivatives
of `TMath::LnGamma` and the related gamma funcitons that are used to
define Poisson cdfs.