use Struct with custom datatype Gitlabrunner::Settings

Because this provide more constraints, this is less error prone:

type Gitlabrunner::Settings = Struct[{
  registration-token => String[1],
  tags               => Optional[Array],
  run-untagged       => Boolean,
  url                => Stdlib::HTTPSUrl,
  concurrent         => Integer,
  maximum-timeout    => Integer,
  limit              => Integrer,
  executor           => Enum['docker','shell'],
  executor-options   => Hash,
}]

Some of them can be Optional like tags. I didn't checked others.

Changing this can produce some code refactoring. For instance i didn't cheked if the code uses the assert_type() function.