Refactor StructuredSignable extraction

[lcarva]

Changes

This change makes the extraction of StructuredSignable objects more flexible so it can handle upcoming requirements such as SBOM support.

Submitter Checklist

As the author of this PR, please check off the items in this checklist:

• Has Docs included if any changes are user facing
• Has Tests included if any functionality added or changed
• Follows the commit message standard
• Meets the Tekton contributor standards (including
  functionality, content, code)
• Release notes block below has been updated with any user facing changes (API changes, bug fixes, changes requiring upgrade notices or deprecation warnings)
• Release notes contains the string "action required" if the change requires additional action from users switching to the new release

Release Notes

NONE

[tekton-robot]

The following is the coverage report on the affected files.
Say /test pull-tekton-chains-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/artifacts/signable.go	73.8%	70.2%	-3.6
pkg/artifacts/structured.go	Do not exist	96.3%

[chitrangpatel]

/lgtm

[chitrangpatel]

/approve

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: chitrangpatel

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