Add docker cred helper adapter in pkg/authn

[imjasonh]

See #1225 (comment)

This adapts an implementation of Docker's credentials.Helper to be used as an authn.Keychain, e.g.:

kc := authn.NewMultiKeychain(
    authn.DefaultKeychain,
    google.Keychain,
    authn.NewKeychainFromHelper(ecr.ECRHelper{ClientFactory: api.DefaultClientFactory{}}),
    authn.NewKeychainFromHelper(credhelper.NewACRCredentialsHelper()),
    ...
)

[codecov-commenter]

Codecov Report

Merging #1227 (d3e6457) into main (7adcade) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1227      +/-   ##
==========================================
+ Coverage   73.76%   73.79%   +0.02%     
==========================================
  Files         111      111              
  Lines        8181     8188       +7     
==========================================
+ Hits         6035     6042       +7     
  Misses       1549     1549              
  Partials      597      597              

Impacted Files	Coverage Δ
pkg/authn/keychain.go	81.25% <100.00%> (+2.30%)	⬆️
pkg/v1/remote/transport/error.go	100.00% <0.00%> (ø)

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