kctrl: Add secret flag for package repo add/update

[devanshuVmware]

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #

1. Add a package repository with secret:
   Given I have a custom package repository,
   And it is hosted on authenticated registry,
   When I add a package repository
   Then I should be able to specify a secret for registry
   kctrl package repository add -r sample-repo --namespace ns --url projects.repo.com/example:1.0.0 --secret repo-secret
   And I execute:
   kubectl get pkgr sample-repo -oyaml
   And I see:

apiVersion: packaging.carvel.dev/v1alpha1
kind: PackageRepository
metadata: 
  name: sample-repo
  namespace: ns
spec: 
  fetch: 
    imgpkgBundle: 
      image: projects.repo.com/example:1.0.0
      - secretRef: 
          name: repo-secret

2. Update a package repository with secret:
   Given I have a package repository created,
   When I update the secret for repository
   kctrl package repository update sample-repo --secret repo-secret-2
   And I run
   kubectl get pkgr sample-repo -oyaml
   Then I see:

apiVersion: packaging.carvel.dev/v1alpha1
kind: PackageRepository
metadata: 
  name: sample-repo
  namespace: ns
spec: 
  fetch: 
    imgpkgBundle: 
      image: projects.repo.com/example:1.0.0
      - secretRef:   
          name: repo-secret-2

Does this PR introduce a user-facing change?

Update upstream kctrl CLI to refer to a secret while adding or updating package repository

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.:

[rohitagg2020]

LGTM