Match the public and private APIs for content reading

[icidasset]

Summary

I've made it so that the public and private APIs for reading content better match.

wnfs changes:

• Made the length param optional and renamed the offset param to match the public side.
• Copied the doc test from the public side to the private side.
• Made it so that PrivateFile.get_content uses its read_at method.
• Added PublicFile.get_content to reflect the private side.

wnfs-wasm changes:

• Added PrivateFile.read_at
• Made it so that PrivateFile.get_content uses its read_at method.
• Added PublicFile.get_content
• Adjusted tests to use new and changed methods.

Test plan (required)

Added and adjusted some tests.

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: 9 lines in your changes are missing coverage. Please review.

Comparison is base (0964ac4) 54.80% compared to head (da90280) 54.57%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #386      +/-   ##
==========================================
- Coverage   54.80%   54.57%   -0.23%     
==========================================
  Files          57       57              
  Lines        3936     3943       +7     
  Branches      947      948       +1     
==========================================
- Hits         2157     2152       -5     
- Misses       1169     1180      +11     
- Partials      610      611       +1     

Files	Coverage Δ
wnfs/src/public/file.rs	69.82% <ø> (ø)
wnfs/src/private/file.rs	70.20% <70.96%> (-3.49%)	⬇️

... and 1 file with indirect coverage changes

[matheus23]

I'd take this as-is, but while "we have the hood up" and working towards public & private APIs being aligned, would you be up for tackling a small refactor from usize to u64 for byte offsets? (See the comment)