Generic data directory for `arecibo`

[winston-h-zhang]

The issue addresses multiple current active issues in that require some kind of external data management from the disk or filesystem:

1. https://github.com/lurk-lab/arecibo/issues/270
2. https://github.com/lurk-lab/lurk-rs/issues/1086
3. https://github.com/lurk-lab/arecibo/pull/335

Regarding this comment: I include https://github.com/lurk-lab/arecibo/pull/335 as one of the use cases because:

1. We've needed it's functionality many times now, as a way to reproduce MSM data for Ingonyama, SN, and most recently spmvm test vectors.
2. I believe we need some way of generating test vectors from our stack, that is easily accessible and merged into the main branch as a maintained API.
3. I don't know what exactly the design should look like, but it will also require accessing a data directory, which is why I include it as part of this issue.

Then the two questions I have for discussion is:

1. How should we unify these 3 topics into a single generic way of managing external data within arecibo?
2. Is this generic approach the correct way to proceed?

cc: @huitseeker