build: build external plugins conditionally

[binbin-li]

Description

What this PR does / why we need it:

1. Update Dockerfile so that it can select which external plugin to be built. The default building will just include notation verifier.
2. Split test.bats to base_test and plugin_test. base_test will cover test cases running on image with built-in notation verifier. plugin_test will cover test cases for each added external plugin. Notes: the original test will be deleted in a following PR.

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 #858

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?

e2e test passed.

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

Patch and project coverage have no change.

Comparison is base (72640d5) 54.96% compared to head (a6a5e65) 54.96%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #860   +/-   ##
=======================================
  Coverage   54.96%   54.96%           
=======================================
  Files          80       80           
  Lines        4739     4739           
=======================================
  Hits         2605     2605           
  Misses       1850     1850           
  Partials      284      284           

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[susanshi]

thanks for the PR , in general looks good, left some minor comments.