feat/nvfetcher blocktype

[blaggacao]

• feat: add nvfetcher blocktype
• test: update snapshot with nvfetcher blocktype

Context

Flake inputs don't store metadata, such as for example the url from which they where fetched.

There's no practical way * to get that data unless the repository contains it in its files.

However, sometisem you need that metadata, for example to clone local compies of the sources to work on them.

* you could parse flake.nix manually with something like (import <reporoot>/flake.nix).inputs, in all theroy

Also, albeit solutions such as flakehub exist, flake inputs are notoriously bad at detecting new versions.

Proposed Solution

Implement an nvfetcher block type in order to:

• Manage app sources
• Manage updates
• Improve their version control
• Access source metadata within your nix code