added keeper secrets manager backend

[robbert229]

Description

This PR adds Keeper Secrets Manager as a supported backend for argocd-vault-plugin. I have implemented, and tested the main use cases, but have not implemented support for versions, annotations, or keeper secrets manager's file attachment system. It does however support most of the types of secrets you commonly want to get out of KSM.

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=1.17 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

N/A

[codecov-commenter]

Codecov Report

Merging #468 (85ffb2d) into main (8043640) will increase coverage by 0.16%.
The diff coverage is 80.00%.

❗ Current head 85ffb2d differs from pull request most recent head e7ef35d. Consider uploading reports for the commit e7ef35d to get more accurate results

@@            Coverage Diff             @@
##             main     #468      +/-   ##
==========================================
+ Coverage   77.36%   77.53%   +0.16%     
==========================================
  Files          23       24       +1     
  Lines        1498     1598     +100     
==========================================
+ Hits         1159     1239      +80     
- Misses        252      268      +16     
- Partials       87       91       +4     

Impacted Files	Coverage Δ
pkg/config/config.go	83.78% <75.00%> (-0.51%)	⬇️
pkg/backends/keepersecretsmanager.go	80.68% <80.68%> (ø)

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[werne2j]

We don't know Keeper secret manager very well, so trusting the implementation.