Increase usage of TektonObject

[lcarva]

Changes

The first commit removes certain parts of the code that switch on the underlying TektonObject type to determine if either the string taskrun or pipelinerun should be used. Instead, a new method on TektonObject is used.

The second commit changes the code to no longer require callers to inspect the underlying type of the TektonObject to determine which Signable is supported. Instead, add a set of methods that specify which artifacts are supported. An alternative approach was consider to introduce a method such as TektonObject.GetSignables to return []artifacts.Signable. However, Signable already has a dependency on TektonObject causing a circular import.

Relates to #875

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/chains/objects/objects.go	40.5%	38.6%	-1.8
pkg/chains/storage/grafeas/grafeas.go	77.6%	78.2%	0.6

[lcarva]

@chitrangpatel, let me know what you think of this approach. It's a small step towards making it easier to support multiple versions of TaskRun and PipelineRun objects.

[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/chains/objects/objects.go	40.5%	34.0%	-6.5
pkg/chains/signing.go	72.0%	73.7%	1.7
pkg/chains/storage/grafeas/grafeas.go	77.6%	78.2%	0.6

[chitrangpatel]

Thanks @lcarva!!! This is amazing!

[chitrangpatel]

Wondering if we can bump up the coverage of objects.go. Otherwise, lgtm.

[lcarva]

Wondering if we can bump up the coverage of objects.go. Otherwise, lgtm.

Great catch! Added a new commit that covers all the functionality added, as well as some existing methods.

[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/chains/objects/objects.go	40.5%	66.0%	25.5
pkg/chains/signing.go	72.0%	73.7%	1.7
pkg/chains/storage/grafeas/grafeas.go	77.6%	78.2%	0.6

[chitrangpatel]

/approve

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: chitrangpatel

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[chitrangpatel]

/lgtm