Add support for taskRun to expose digest of built images

 - This features requires the builder used to build the image to export
 an oci standard layout file `index.json`
 https://github.com/opencontainers/image-spec/blob/master/image-layout.md
 - If the `index.json` file is not present, the digest will not be
 exposed in the taskRun
 - add a field in the image resource to indicate the location of the
 index.json file -> `indexpath`
 - add a field in the taskRun status to include the image name and
 digest -> `resourcesResult`
 - add a container step to collect the digest from all the files
 - the taskRun controller will collect the output from the
 image-digest-exporter container and update the taskRun status