Extract shared code between x402-gateway and piece-retrieval workers

[pyropy]

Let's extract this into a shared helper (see https://github.com/filbeam/worker/tree/main/retrieval/lib).

It's ok to do that cleanup later, after this PR lands.

Originally posted by @bajtos in #523 (comment)

[FAIZ1409]

Hi 👋
I’d like to work on this issue if it’s still available.
Please let me know if I can proceed or if there are any specific guidelines to follow. Thanks!

[bajtos]

@FAIZ1409, sounds great!

• We have another retrieval worker in the making (Serve IPFS retrievals #312). Some of the code shared between the x402-retriever and piece-retriever will be shared with ipfs-retriever as well. Please take that into consideration.
• To make it easier for us to review your pull requests, please open multiple smaller pull requests. For example, start by extracting a single shared helper function. After that change is reviewed & landed, open another pull request for more changes.