feat: Add a function to get the exact size of a PublicFile and PrivateFile

[icidasset]

Summary

Adds a get_size function to PublicFile and PrivateFile, and their wasm equivalents, that returns the exact size of the contents without downloading all the content blocks.

Test plan (required)

Added doc tests for the core lib, and tests for the wasm lib.

After Merge

• Does this change invalidate any docs or tutorials? If so ensure the changes needed are either made or recorded
• Does this change require a release to be made? Is so please create and deploy the release

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 54.69%. Comparing base (8ac4021) to head (2eb7ca7).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #405      +/-   ##
==========================================
+ Coverage   54.58%   54.69%   +0.11%     
==========================================
  Files          57       57              
  Lines        3959     3958       -1     
  Branches      959      959              
==========================================
+ Hits         2161     2165       +4     
+ Misses       1183     1175       -8     
- Partials      615      618       +3     

Files	Coverage Δ
wnfs-common/src/metadata.rs	24.52% <ø> (ø)
wnfs-common/src/storable.rs	32.43% <ø> (ø)
wnfs/src/public/file.rs	68.96% <ø> (ø)
wnfs-hamt/src/diff.rs	19.46% <0.00%> (ø)
wnfs/src/private/file.rs	70.11% <85.71%> (-0.09%)	⬇️

... and 2 files with indirect coverage changes

[matheus23]

Thank you for this! Great work 🙏

I'm especially happy to see you've done the work of adding Wasm tests 🙌 so I'm pretty confident in this.