feat: Implement public directory cp & more efficient copy for PrivateFile

[matheus23]

Fixes #316 (although what was missing was actually cp for Public directory, not private)

Since #306 we can actually do a more efficient copy of external PrivateFile content that doesn't require re-encryption, because we can directly refer to the old content (using the bare_name property of ExternalFileContent).

I've decided to set this as the default copy algorithm. There's no benefit to re-encrypting the actual content bytes. You already get the key rotation on the wrapping PrivateFile, and it doesn't implicitly give you access to anything else besides the bytes you're supposed to be able to read.

I think this supersedes #256 at least in terms of what Banyan needs.

[codecov]

Codecov Report

Merging #319 (7f15ee6) into main (55cf2e0) will increase coverage by 0.02%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##             main     #319      +/-   ##
==========================================
+ Coverage   56.28%   56.30%   +0.02%     
==========================================
  Files          43       43              
  Lines        3207     3204       -3     
  Branches      775      773       -2     
==========================================
- Hits         1805     1804       -1     
  Misses        904      904              
+ Partials      498      496       -2     

Files Changed	Coverage Δ
wnfs/src/private/directory.rs	67.78% <ø> (ø)
wnfs/src/private/node/node.rs	69.94% <0.00%> (-0.18%)	⬇️
wnfs/src/public/directory.rs	70.42% <ø> (ø)
wnfs/src/private/file.rs	78.91% <100.00%> (+0.53%)	⬆️

[appcypher]

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