Remove filesystem operations from the backend

[phated]

Problem

Currently, this backend manages caching of its CRS, but it writes to a filepath that should be maintained by Nargo.

Proposed solution

We should change the backend trait to return a CRS as bytes and Nargo can manage caching it. This will ensure that all CRS from various backends are handled in a consistent manner.

Alternatives considered

No response

Additional context

No response

Submission Checklist

• Once I hit submit, I will assign this issue to the Project Board with the appropriate tags.