add crane auth logout

[imjasonh]

Fixes #1587

NB: This doesn't clear creds from Podman-configured auth, which authn.DefaultKeychain uses if a Docker config file isn't found. This might be confusing, but crane auth login doesn't use that either, so 🤷.

It might make more sense to always login using DOCKER_CONFIG and logout of Docker- or Podman-configured auth, feedback welcome.

[codecov-commenter]

Codecov Report

Merging #1589 (89caef3) into main (ff810c1) will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1589      +/-   ##
==========================================
+ Coverage   72.88%   72.91%   +0.03%     
==========================================
  Files         118      118              
  Lines        9422     9422              
==========================================
+ Hits         6867     6870       +3     
+ Misses       1861     1859       -2     
+ Partials      694      693       -1     

Impacted Files	Coverage Δ
pkg/v1/remote/write.go	62.84% <0.00%> (+0.41%)	⬆️

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[imjasonh]

Some things I've learned about the Erase method, and by extension, docker logout:

• it doesn't unset cred helpers for the server
• it doesn't complain if creds already aren't defined for the server

I think I'd like to fix both of those for crane auth logout, but I'd be willing to be convinced that following docker's behavior exactly is better.