Limit client libraries for external usage

[aaron-prindle]

This issue tracks the work required to limit tektoncd/chains usage as a client library. Currently some methods/structs are exposed to users that we don't want to be public (eg: don't want them to depend-on/use). Work required is likely a refactor to make some currently public methods, interfaces, structs, etc. private

[aaron-prindle]

Analysis on tektoncd/chains usage as a client library externally. The below methods are those that are relied on for client library external usage to be considered/hopefully-kept-available when limiting client libraries:

"github.com/tektoncd/chains/pkg/chains/formats/formats”

• formats.GetPayloader
• formats.PayloadTypeSlsav2alpha3

"github.com/tektoncd/chains/pkg/chains/objects/objects"

• objects.TektonObject
• objects.NewPipelineRunObjectV1
• objects.NewTaskRunObjectV1

"github.com/tektoncd/chains/pkg/config/config"

• config.Config
• config.BuilderConfig
• config.ArtifactConfig
• config.BuildDefinitionConfig

"github.com/tektoncd/chains/pkg/chains/formats/slsa/extract/extract"

• extract.RetrieveAllArtifactURIs

"github.com/tektoncd/chains/pkg/chains/formats/slsa/v2alpha3/v2alpha3"

• Imported as underscore “_” for side-effects:
  • Init function here - https://github.com/tektoncd/chains/blob/main/pkg/chains/formats/slsa/v2alpha3/slsav2.go#L35-L37