Add resumable download support via range requests

[ericcurtin]

Add documentation and comprehensive tests for resumable downloads

Add resumable download example and update gitignore

[ericcurtin]

This is important, it is getting common for people to put AI models in layers. They can be many GBs in size increasing the chances of interrupted downloads being problematic.

[ericcurtin]

HI @brandonmichigangithub @Subserial

I'd appreciate it if you could take a look at this PR when you have a moment. As mentioned, resumable downloads are becoming increasingly important for large container layers, like those used for AI models, where download interruptions are more likely.

I've also noticed that the PR queue is quite long and the merge velocity seems low. I completely understand this is common for popular and busy projects. This got me thinking about the project's long-term bandwidth.

Has the team considered adding more co-maintainers from the community to help manage the backlog of issues and PRs? There are likely several people (myself included) who would be willing to step up, share the load, and help keep the project moving.

Thanks for your time and for maintaining this project!

[codecov-commenter]

Codecov Report

❌ Patch coverage is 39.62264% with 32 lines in your changes missing coverage. Please review.
✅ Project coverage is 52.98%. Comparing base (8b3c303) to head (9cd0eb4).
⚠️ Report is 41 commits behind head on main.

Files with missing lines	Patch %	Lines
examples/resumable-download/main.go	0.00%	22 Missing ⚠️
pkg/v1/remote/layer.go	50.00%	3 Missing and 3 partials ⚠️
pkg/v1/remote/fetcher.go	78.94%	2 Missing and 2 partials ⚠️

❗ There is a different number of reports uploaded between BASE (8b3c303) and HEAD (9cd0eb4). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (8b3c303)	HEAD (9cd0eb4)
	2	1

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2144       +/-   ##
===========================================
- Coverage   71.67%   52.98%   -18.70%     
===========================================
  Files         123      165       +42     
  Lines        9935    10987     +1052     
===========================================
- Hits         7121     5821     -1300     
- Misses       2115     4455     +2340     
- Partials      699      711       +12     

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