refactor: Rename vault to backends, start using AuthType interface

[werne2j]

Description

• Refactor the code to have a simpler structure as well as reuse code by taking advantage of more interfaces.
• Remove the vault client and use the vault api client directly
• Pull config and types into own package

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
• Docs have been added / updated

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)

[codecov]

Codecov Report

Merging #59 (e95d7b4) into main (5fe0e85) will increase coverage by 1.15%.
The diff coverage is 43.47%.

@@            Coverage Diff             @@
##             main      #59      +/-   ##
==========================================
+ Coverage   44.73%   45.89%   +1.15%     
==========================================
  Files          13       14       +1     
  Lines         427      438      +11     
==========================================
+ Hits          191      201      +10     
+ Misses        213      209       -4     
- Partials       23       28       +5     

Impacted Files	Coverage Δ
cmd/generate.go	35.55% <0.00%> (ø)
cmd/root.go	0.00% <0.00%> (ø)
pkg/auth/ibmsecretmanager/iam.go	0.00% <0.00%> (ø)
pkg/auth/vault/github.go	0.00% <0.00%> (ø)
pkg/kube/template.go	48.33% <0.00%> (ø)
pkg/backends/ibmsecretmanager.go	56.25% <56.25%> (ø)
pkg/auth/vault/approle.go	71.42% <71.42%> (ø)
pkg/backends/vault.go	76.00% <76.00%> (ø)
pkg/config/config.go	86.36% <100.00%> (ø)
pkg/utils/util.go	55.00% <100.00%> (ø)
... and 6 more

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[jkayani]

A Vault configuration has a Backend which can use one of several AuthType's depending on which specific backend it is. Each Backend encapsulates the logic for getting the data into the map[string]interface{} from it's Vault (plain Vault or SecretManager). The AuthType's handle logging in and setting the Vault token in the client.

Great change I think. I didn't read the details too closely so I will rely on your tests and the CI to make sure it's good