feat: Introduce update strategy 'digest'

[jannfis]

This change introduces the new update strategy digest, which allows to track a single tag for updates.

Configuration is as follows to track new versions of latest tag pushed to the some/image repository:

argocd-image-updater.argoproj.io/image-list: foo=some/image:latest
argocd-image-updater.argoproj.io/foo.update-strategy: digest

The tag to track must be specified in the constraint section of the image list.

Closes #174

Signed-off-by: jannfis [email protected]

[codecov]

Codecov Report

Merging #176 (af78293) into master (5da428b) will decrease coverage by 1.31%.
The diff coverage is 46.98%.

@@            Coverage Diff             @@
##           master     #176      +/-   ##
==========================================
- Coverage   68.37%   67.05%   -1.32%     
==========================================
  Files          19       19              
  Lines        1404     1454      +50     
==========================================
+ Hits          960      975      +15     
- Misses        358      388      +30     
- Partials       86       91       +5     

Impacted Files	Coverage Δ
pkg/image/options.go	73.86% <0.00%> (-1.72%)	⬇️
pkg/registry/client.go	0.00% <0.00%> (ø)
pkg/argocd/update.go	68.45% <20.00%> (-1.00%)	⬇️
pkg/registry/registry.go	59.81% <29.41%> (-1.36%)	⬇️
pkg/image/version.go	62.06% <31.25%> (-14.02%)	⬇️
pkg/tag/tag.go	83.87% <63.63%> (-2.93%)	⬇️
pkg/image/image.go	92.68% <95.23%> (-1.84%)	⬇️
pkg/argocd/argocd.go	60.30% <100.00%> (ø)

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