add extended generic section

[celek]

End-user friendly description of the problem this fixes or functionality that this introduces

• Include this change in the Release Notes. If checked, you must provide an end-user friendly description for your change below

---

Give a summary of what the PR does, explaining any non-trivial design decisions
Creates a dynamic generic 'extended' section in config.toml

[extended]
key_1 = value_1
key_2 = value_2

you can access the value using:

value = cfg.extended.key_1

or

value = cfg.extended["key_2"]

---

Link of any specific issues this addresses
#5849

[celek]

@enyst as you may have noticed, I am not very good in Git (PR, Commit) nor an expert in python - but I think this PR is a bit better 🤞

[enyst]

Awesome, thank you! I am still to see how it works, but I'll get back to it!