Add staging pipeline support to update-dependencies tool

[lbussell]

• New command: from-staging-pipeline
  • This command supplants Get-DropVersions.ps1 -BuildId ... and accomplishes the same things.
  • This command does not yet update dependencies with internal versions. This means the base urls won't be updated and the Dockerfiles won't be generated with the internal versions of the templates just yet.
  • The reason for that is the old update-dependencies code depends on using the base URL for accessing the sha files, so that will need to be updated or replaced separately.