feat: add PolicyManager interface to wrap operations on namespaced policies [multi-tenancy PR 3]

[binbin-li]

Description

What this PR does / why we need it:

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

1. Add a PolicyManager interface to wrap operations on namespaced policy resources.
2. Update policy references to use the new PolicyManager interface instead of directly accessing policies.
3. Move the ActivePolicies under controllers to resource_map.go so that it can be shared by clustered and namespaced controllers in the future.
4. Updated unit tests. Some unit tests are not added in this PR as we'll change the behavior soon.
5. 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 75.00000% with 7 lines in your changes are missing coverage. Please review.

Project coverage is 65.26%. Comparing base (73ef709) to head (00a5b40).
Report is 6 commits behind head on staging.

Files	Patch %	Lines
pkg/manager/manager.go	0.00%	3 Missing ⚠️
pkg/controllers/policy_controller.go	33.33%	2 Missing ⚠️
pkg/executor/core/executor.go	0.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           staging    #1359      +/-   ##
===========================================
- Coverage    65.31%   65.26%   -0.06%     
===========================================
  Files          105      106       +1     
  Lines         5458     5533      +75     
===========================================
+ Hits          3565     3611      +46     
- Misses        1535     1567      +32     
+ Partials       358      355       -3     

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[akashsinghal]

lgtm