Missing public interfaces for `ParsedArgs`

[christiangnrd]

#207 broke CUDA tests because CUDA explicitly constructs a ParsedArgs object, and I found a few more places where we need a public interface so users aren't accessing ParsedArgs fields directly.

• Custom arguments: Packages currently need to access custom arguments by args.custom[<key>], we should maybe define a get_custom_arg(<key>) function.
• Number of jobs: Our recent changes are a step in the right direction or cpu-only tests, but sometimes the number of workers needs information based on other attributes. Some packages push a string to the unparsed argument vector, while CUDA, recreates a ParsedArgs object. Maybe we could allow packages to pass in a closure to determine the number of jobs?