[RT-2857] Allow users to define super-global arguments that are injected into all steps

[blu3r4y]

Acceptance Criteria:

• There is a config file next to Invadium where I can define "super-global" arguments that will be passed into every container and every step, always
  • An example would be the TARGET_URL or something else that is needed by all exploits
• Those arguments shall be read-only and not available to be changed

Notes:

It is probably best to re-structure the folder structure so that I only have to mount one volume that has everything in it, here is an example (inspired by GitLab):

.
├── config
│   └── config.yaml
└── exploits
    ├── cmdi.yaml
    └── sqli.yaml

Tasks:

• Possibly think about a better name to easily distinguish super-global / global / local arguments
• Load super-global arguments in the backend
• Update the README and explain the new folder structure
• Ensure that global and local arguments with the same name override super-global ones
• Show the super-global arguments in the frontend, but don't let the user modify them
• Use super-global arguments in one of the existing exploits
• Test that I can still override them when I use the same name in the global or local scope of an exploit