fix: dynamic plugin should support pulling image with digest

[susanshi]

Description

What this PR does / why we need it:

The download logic uses the source object to form the blob reference. However when a digest is used, the reference becomes malformed as the regex that looks for the blob reference would parse out the digest.

When a tag is passed:

{wabbitnetworks.azurecr.io/test/sample-store-plugin:v1 map[]}@sha256:26d320ceaa2d027ea2bceaae36a82e304782bbbbe92c8f0b1102482c596f32f5 (blob reference)

When a digest is passed:

{wabbitnetworks.azurecr.io/test/sample-store-plugin:294c517b9432a59f2d829f6d04912833a256560e4128aca86efa0c6b4ba6d44 map[]}@sha256:26d320ceaa2d027ea2bceaae36a82e304782bbbbe92c8f0b1102482c596f32f5 (blob reference)

@binbin-li followed up with Oras folks to confirm only the reference is required. This PR updates the code path to ONLY pass the blob to the oras library.

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 #1278

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

Locally tested with dynamic plugin both using tag and digest

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

Comparison is base (2be797d) 61.72% compared to head (b998880) 61.69%.

Files	Patch %	Lines
pkg/common/plugin/download.go	40.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1280      +/-   ##
==========================================
- Coverage   61.72%   61.69%   -0.04%     
==========================================
  Files          97       97              
  Lines        6190     6192       +2     
==========================================
- Hits         3821     3820       -1     
- Misses       2038     2040       +2     
- Partials      331      332       +1     

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[akashsinghal]

great catch. Thanks for the fix!

[binbin-li]

lgtm overall, thanks for the fix！

[junczhu]

LGTM, thanks for the fix！