command added to list-packages from package repository

[rcmadhankumar]

What this PR does / why we need it:

Currently we cannot list the available packages in Package Repository without deploying PackageRepository CR and all Package CRs from Package Repository resources on a cluster. In this PR, list-packages subcommand is added to kctrl's package repository command to list the available packages from a package repository without deploying them on the cluster.

Output:

$kctrl package repository list-packages --url index.docker.io/k8slt/kc-e2e-test-repo:latest              

$ imgpkg pull -b index.docker.io/k8slt/kc-e2e-test-repo:latest -o ./bundle-index.docker.io-k8slt-kc-e2e-test-repo:latest-2648073237 --tty=true

Pulling bundle 'index.docker.io/k8slt/kc-e2e-test-repo@sha256:ddd93b67b97c1460580ca1afd04326d16900dc716c4357cade85b83deab76f1c'
  Extracting layer 'sha256:a18d039a28a58cba06a4aaf435e4479b88daeb3cf7a84e1ca1a5d913f56ad1a8' (1/1)

Locating image lock file images...
One or more images not found in bundle repo; skipping lock file update

Succeeded


Listing Packages from Package Repository 'index.docker.io/k8slt/kc-e2e-test-repo:latest'

Name                 Version  
pkg.test.carvel.dev  1.0.0  
^                    2.0.0  
^                    3.0.0-rc.1  

Succeeded

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?

NONE

Additional Notes for your reviewer:

Review Checklist:

• Follows the developer guidelines
• Relevant tests are added or updated
• Relevant docs in this repo added or updated
• Relevant carvel.dev docs added or updated in a separate PR and there's
  a link to that PR
• Code is at least as readable and maintainable as it was before this
  change

Additional documentation e.g., Proposal, usage docs, etc.: