feat: add verifiers interface to wrap up operations on namespaced verifiers [multi-tenancy PR 2]

[binbin-li]

Description

What this PR does / why we need it:

This is the second PR for multi-tenancy support. Please review this PR first.

1. Add a VerifierManager interface to wrap operations on the namespaced verifier resources.
2. Update verifier references to use the new Verifiers interface instead of directly accessing the verifier slice.
3. Move the VerifierMap variable under controllers to a separate file so that both clustered and namespaced controllers could share it in the future.
4. Add context param to the getExecutor method so that Ratify could fetch the corresponding verifiers/stores/policies per namespace during initializing executor.
5. Updated unit tests. Some unit tests are not added in this PR as we'll change the behavior soon.
6. For the interface implementation, it's in a transition stage that keeps the old behavior. Will update the implementation once Ratify fully supports multi-tenancy.

Which issue(s) this PR fixes (optional, using fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when the PR gets merged):

Fixes #

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Helm Chart Change (any edit/addition/update that is necessary for changes merged to the main branch)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Please also list any relevant details for your test configuration

• Test A
• Test B

Checklist:

• Does the affected code have corresponding tests?
• Are the changes documented, not just with inline documentation, but also with conceptual documentation such as an overview of a new feature, or task-based documentation like a tutorial? Consider if this change should be announced on your project blog.
• Does this introduce breaking changes that would require an announcement or bumping the major version?
• Do all new files have appropriate license header?

Post Merge Requirements

• MAINTAINERS: manually trigger the "Publish Package" workflow after merging any PR that indicates Helm Chart Change

[codecov]

Codecov Report

Attention: Patch coverage is 81.63265% with 9 lines in your changes are missing coverage. Please review.

❗ No coverage uploaded for pull request base (staging@034f5ec). Click here to learn what that means.

Files	Patch %	Lines
config/configManager.go	0.00%	4 Missing ⚠️
httpserver/handlers.go	87.50%	0 Missing and 2 partials ⚠️
pkg/manager/manager.go	0.00%	2 Missing ⚠️
pkg/controllers/verifier_controller.go	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
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  Coverage           ?   65.31%           
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  Files              ?      105           
  Lines              ?     5458           
  Branches           ?        0           
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  Hits               ?     3565           
  Misses             ?     1535           
  Partials           ?      358           

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[susanshi]

thanks for the PR Binbin! left some clarification questions.

[susanshi]

LGTM. thanks Binbin!