Dispersion in latent_ll_oscale() and latent_ppd_oscale()

[fweber144]

This adds/requires an argument dis to/in latent_ll_oscale() functions and an argument dis_resamp to/in latent_ppd_oscale() functions. This is relevant for a log-normal response family and emerged out of https://discourse.mc-stan.org/t/using-projpred-latent-projection-with-brms-weibull-family-models/39275/4.

A NEWS.md entry is still missing here. I will add it as soon as #511 (or #508) has been merged. I would propose the following text for such a NEWS.md entry (under "Major changes" because this is a breaking change for users with custom latent_ll_oscale() or latent_ppd_oscale() functions):

* For the latent projection, the function passed to argument `latent_ll_oscale` of `extend_family()` now needs to have an argument `dis` (at the second position). Similarly, the function passed to argument `latent_ppd_oscale` of `extend_family()` now needs to have an argument `dis_resamp` (again at the second position). This makes it possible, e.g., to use the latent projection for a log-normal response family. (GitHub: #513)