fix: Enforce validation on notation signature blob number

[binbin-li]

Description

What this PR does / why we need it:

1. A quick fix to enforce validation on the Notation signature blob number. There should be exactly 1 blob in the manifest.Blobs which is also validated in Notation-go: https://github.com/notaryproject/notation-go/blob/8e2131d192fdb71c5268f81b4b9b93069559d3b5/registry/repository.go#L198
2. Remove critical cache failure in oras GetReferenceManifest

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 95.83333% with 1 line in your changes missing coverage. Please review.

Files	Patch %	Lines
pkg/referrerstore/oras/oras.go	85.71%	1 Missing ⚠️

Files	Coverage Δ
pkg/referrerstore/oras/mocks/oras_storage.go	93.33% <100.00%> (+2.42%)	⬆️
pkg/verifier/notation/notation.go	91.39% <100.00%> (+3.27%)	⬆️
pkg/referrerstore/oras/oras.go	71.66% <85.71%> (+3.88%)	⬆️

... and 11 files with indirect coverage changes

[akashsinghal]

LGTM. thanks for double checking spec again