feat: add new subcommand skopeo-sync-gen

[djzager]

This new subcommand will parse imagestreams, looking for a local image and add it to the "source" list that can be passed to skopeo sync for syncing the images to a new cluter.

[openshift-ci[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: djzager

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Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/konveyor/crane/blob/main/OWNERS)~~ [djzager] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[djzager]

Testing manually:

$ tree export
export
├── failures
│   └── test-source
└── resources
    └── test-source
        ├── ConfigMap_test-source_kube-root-ca.crt.yaml
        ├── ConfigMap_test-source_openshift-service-ca.crt.yaml
        ├── ImageStreamTag_test-source_alpine:latest.yaml
        ├── ...
        ├── ImageStream_test-source_alpine.yaml
        ├── ImageStream_test-source_busybox.yaml
        ├── ImageStream_test-source_golang.yaml
        ├── ImageStream_test-source_httpd.yaml
        ├── ImageStream_test-source_internal-alpine.yaml
        ├── ImageStream_test-source_internal-bar.yaml
        ├── ImageStream_test-source_internal-baz.yaml
        ├── ImageStream_test-source_internal-busybox.yaml
        ├── ImageStream_test-source_internal-corge.yaml
        ├── ImageStream_test-source_internal-foo.yaml
        ├── ImageStream_test-source_internal-fred.yaml
        ├── ImageStream_test-source_internal-garply.yaml
        ├── ImageStream_test-source_internal-grault.yaml
        ├── ImageStream_test-source_internal-plugh.yaml
        ├── ImageStream_test-source_internal-quux.yaml
        ├── ImageStream_test-source_internal-quuz.yaml
        ├── ImageStream_test-source_internal-qux.yaml
        ├── ImageStream_test-source_internal-waldo.yaml
        ├── ImageStream_test-source_mongo.yaml
        ├── ImageStream_test-source_nextcloud.yaml
        ├── ImageStream_test-source_ruby.yaml
        ├── ImageStream_test-source_tomcat.yaml
        ├── ...

4 directories, 84 files

$ ./crane skopeo-sync-gen --export-dir ./export --registry-url $registry
default-route-openshift-image-registry.djzager-vpc1-3626522b15eedb880d7b99992e225c1b-0000.us-east.containers.appdomain.cloud:
  images:
    test-source/internal-alpine: []
    test-source/internal-bar: []
    test-source/internal-baz: []
    test-source/internal-busybox: []
    test-source/internal-corge: []
    test-source/internal-foo: []
    test-source/internal-fred: []
    test-source/internal-garply: []
    test-source/internal-grault: []
    test-source/internal-plugh: []
    test-source/internal-quux: []
    test-source/internal-quuz: []
    test-source/internal-qux: []
    test-source/internal-waldo: []

Was able to confirm this working with skopeo sync --src yaml ${output_from_skopeo-sync-gen}.

[jmontleon]

/lgtm