Breaking: Add context to ArgLoader function, nil_is_not_found modifier, nested keys feature and improve errors/doc

[tofran]

Load function is now:

load_function: fn _context, id ->
  get_user_by_id(id)
end

Also added :nil_is_not_found opt

[Copilot]

Copilot reviewed 1 out of 2 changed files in this pull request and generated no comments.

Files not reviewed (1)

• lib/middleware/arg_loader.ex: Language not supported