[Proposal] Pkg extensions for dedicated sampler

[szabo137]

There might be dedicated samplers for specific processes implemented in QEDprocesses.jl, e.g. Kahn-Sampler for perturbative Compton scattering. However, since we decided to keep the development of QEDprocesses.jl and QEDevents.jl orthogonal, QEDprocesses.jl must not become a dependence. Nevertheless, to implement such type of a sampler, one only needs the types of process, model, and phase-space definition for multiple dispatch. So, I would suggest using package extensions as a solution to this problem. This means, that every sampler that is dedicated to a process given in QEDprocesses.jl should be placed in ext and should use QEDprocesses.jl as a weak dependence. The result is, that this code is loaded, only if QEDprocesses.jl is loaded as well, which must be the case if the sampler input is a process from there in the first place.