feat: add skip namespace validation option for RBAC filters

[sleeyax]

What this PR does / why we need it?

Split from #563.

Currently, there's a hard-coded validation check in the code to verify that the supplied namespaces exist. However, clusters with limited permissions may not have the permission to list all namespaces. I added a flag to optionally disable this check for such cases.

PR Checklist

• This PR adds K8s exceptions (false positives)
• This PR adds new code
• This PR includes tests for new/existing code
• This PR adds docs

GitHub Issue

Not applicable (I think). Feel free to create one retroactively.

[codecov-commenter]

⚠️ Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

❌ Patch coverage is 0% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 46.47%. Comparing base (58fc28d) to head (78484d8).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/filters/options.go	0.00%	12 Missing ⚠️
cmd/kor/root.go	0.00%	1 Missing ⚠️
❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #572      +/-   ##
==========================================
- Coverage   46.56%   46.47%   -0.10%     
==========================================
  Files          69       69              
  Lines        3930     3938       +8     
==========================================
  Hits         1830     1830              
- Misses       1794     1802       +8     
  Partials      306      306              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.