Add support for conditional image tarball imports

[brandond]

Proposed Changes

Normally K3s will import all tarballs in the image dir on startup, and re-import any tarballs that are created or changed while it is running.

This change allows users to opt into only importing tarballs that have changed since they were last imported, even across restarts.

This behavior is opted into by touching a .cache.json file in the images dir. This file is used to track across restarts the size and mtime of the image files when they are imported.

Types of Changes

enhancement

Verification

1. mkdir -p /var/lib/rancher/k3s/agent/images/; touch /var/lib/rancher/k3s/agent/images/.cache.json
2. Copy images into images dir
3. Start k3s; note that images are imported
4. Restart k3s; note that images are NOT imported again.
5. Touch image tarball or replace it with a different file; note that it is imported again.

Testing

Existing tests cover airgap image import

Linked Issues

• Allow skipping import of airgap image tarballs that have not changed #12243

User-Facing Change

Further Comments

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 149 lines in your changes missing coverage. Please review.

Project coverage is 19.82%. Comparing base (af51c34) to head (2cb677e).
Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/agent/containerd/watcher.go	0.00%	148 Missing ⚠️
pkg/agent/containerd/containerd.go	0.00%	1 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (af51c34) and HEAD (2cb677e). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (af51c34)	HEAD (2cb677e)
e2etests	1	0

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #12237       +/-   ##
===========================================
- Coverage   40.36%   19.82%   -20.54%     
===========================================
  Files         187      184        -3     
  Lines       19211    19136       -75     
===========================================
- Hits         7754     3794     -3960     
- Misses      10273    14912     +4639     
+ Partials     1184      430      -754     

Flag	Coverage Δ
e2etests	?
unittests	19.82% <0.00%> (+0.01%)	⬆️

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