refactor: more flexible extrinsic unwrapping for ctype fetching

[rflechtner]

fixes https://github.com/KILTprotocol/ticket/issues/3119

While I hot fixed the current behaviour in #836 to allow extrinsics submitted via dispatchAs, I think it's a better strategy for the future to build on top of on an unwrapper function that returns nested calls contained within other extrinsics that can easily be amended. E.g., we could easily add unwrapping of proxy calls to the logic here and everything still works.

How to test:

Ideally, fetch a CType with it.

Checklist:

• I have verified that the code works
• I have verified that the code is easy to understand
  • If not, I have left a well-balanced amount of inline comments
• I have left the code in a better state
• I have documented the changes (where applicable)
  • Either PR or Ticket to update the Docs
  • Link the PR/Ticket here

[ntn-x2]

I like your solution, very elegant. I am wondering if it can be structured a bit better with regards to functions responsibilities. Other than that, I like it a lot!

[ntn-x2]

Left one more minor comment, otherwise it looks good.