Allow users to register bespoke system dependencies

[nuest]

The best option is sysreqs. If that does not work I see this option:

Ask users to create a PR based on a template for bespoke installation functions

containerit::bespoke_instructions <- list()
containerit::bespoke_instructions["myPkg"] <- list(Run(..), Run_shell(..))

Users could also register more complex instructions at runtime, e.g.

register_bespoke_system_deps(packageName = "sf", FUN = function() {...})

See unused function .install_sf_with_outdated_system_deps for an example that is no longer used/required.