[ Feature Request ] File Proxies for Content Deduplication.

**Is your feature request related to a problem? Please describe.**
Currently, I had a PDF Doc(250 MB) with featured Graphics on drive and want to publish on ZeroNet, but I don't want two copies on my computer.

**Describe the solution you'd like**
Solution I want to propose here is, we should have file proxies on sites with some extension, these should contain file location, file hash, file size etc meta data, when ZeroNet saw these it will retrieve meta data and send the proxy to peers and after receival proxy file, other side will check hash and ask the original file. so this way we don't have to maintain 2 copies of file, ZeroNet should also able check for file changes.

**Problems that will solved by these files**
- Duplicate files across sites will be maintained at single place using this technique. This Partially Solves https://github.com/HelloZeroNet/ZeroNet/issues/2192
- Files which are more than certain size will diverted to unified location and serve from there.

**Additional context**
We can also use Hash Section to add Other hashes like those used by torrent protocol or IPFS content address hashes etc, so plugins will handles non default hash types in the file.

No files, tests, or entry points are named in the issue. First investigate how ZeroNet currently transfers and stores site files, then define the proxy metadata and peer verification flow; done should include handling file changes, deduplication across sites, and retrieval of the original file without maintaining duplicate copies.

索引モデルが issue の本文から書いたものです。