Estimate Gaia mags from PFS spectra

During the work of PIPE2D-1582, we realized that getting Gaia fluxes from PFS spectra is tricky because the Gaia filter responses extend to <380nm, where PFS does not cover. There are multiple ways to deal with this, but this ticket is going to explore a simple but physically motivated way to infer the Gaia fluxes from PFS instead of a simple extrapolation of PFS spectra.