feat: support nar hash query parameter

[cafkafk]

Flake inputs will expect being able to query by nar hash:

error: NAR hash mismatch in input 'http://rime.cx/v1/github/semnix/rust-overlay.tar.gz?narHash=sha256-RRIVKv%2BtiI1yn1PqZiVGQ9YlQGZ%2B/9iEkA4rst1QiNk%3D' (/nix/store/vf4w1wz938x8pk7chqrd68hrd21h4vsg-source), expected 'sha256-RRIVKv+tiI1yn1PqZiVGQ9YlQGZ+/9iEkA4rst1QiNk=', got 'sha256-FkwIBTAMsxyceQce0Mbm+/+cOjj2r5IHBK4R/ekPNaw='

There is a bit of exploration into how nix works that can be done here, which seems pretty fun.

Also, whether or not we can actually provide these hashes without having some sort of store is an open question.