feat: add namespace to external data request key

[binbin-li]

Description

What this PR does / why we need it:

Adding a namespace field to the external data request key.
The current key is the image that needs to be validated. For multi-tenancy model, Ratify requires users provide both image and namespace that would be deployed to.
The new key would look like: [namespaceA]testRegistry/testRepo:v1
And this change is backward-compatible, so the original single image is supported.

Futhermore, once we fully support multi-tenancy model, requests without namespaces would be regarded as validation per cluster-wide instead of namespaced.

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):

Part of issue: #743

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: 7 lines in your changes are missing coverage. Please review.

Comparison is base (d64c713) 55.13% compared to head (6377e7e) 55.13%.

Files	Patch %	Lines
httpserver/handlers.go	55.55%	3 Missing and 1 partial ⚠️
pkg/utils/utils.go	70.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1201      +/-   ##
==========================================
- Coverage   55.13%   55.13%   -0.01%     
==========================================
  Files         104      104              
  Lines        6879     6894      +15     
==========================================
+ Hits         3793     3801       +8     
- Misses       2753     2758       +5     
- Partials      333      335       +2     

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[junczhu]

LGTM

[susanshi]

Left a minor comment. thanks!