feat: support placeholders in dockerconfigjson auth field

[teejaded]

Description

Add support for placeholders in dockerconfigjson pull secrets.

Fixes: #152

Checklist

Please make sure that your PR fulfills the following requirements:

• Reviewed the guidelines for contributing to this repository
• The commit message follows the Conventional Commits Guidelines.
• Tests for the changes have been updated
• Are you adding dependencies? If so, please run go mod tidy -compat=1.21 to ensure only the minimum is pulled in.
• Docs have been added / updated
• Optional. My organization is added to USERS.md.

Type of Change

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• New tests
• Build/CI related changes
• Documentation content changes
• Other (please describe)

Other information

Not mentioned in the issue, but this PR also resolves a problem with go marshaled dockerconfigjson. By default golang escapes <>& to mitigate html injection attacks. AVP would typically ignore the placeholders because the characters are replaced with their unicode escape sequence equivalents.

❯ kubectl create secret docker-registry test --docker-server=https://my-server.local --docker-username='<user>' --docker-password='<pass>' -o yaml --dry-run=client | yq '.data.".dockerconfigjson" | @base64d'

{"auths":{"https://my-server.local":{"username":"\u003cuser\u003e","password":"\u003cpass\u003e","auth":"PHVzZXI+OjxwYXNzPg=="}}}

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 74.00000% with 13 lines in your changes missing coverage. Please review.

Project coverage is 71.80%. Comparing base (b2d7f10) to head (b8ea806).
Report is 21 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/kube/util.go	77.27%	6 Missing and 4 partials ⚠️
pkg/kube/template.go	50.00%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #586      +/-   ##
==========================================
+ Coverage   71.75%   71.80%   +0.04%     
==========================================
  Files          26       26              
  Lines        1983     2032      +49     
==========================================
+ Hits         1423     1459      +36     
- Misses        460      467       +7     
- Partials      100      106       +6     

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[miwig]

I would like this feature also, how can I help get this merged?