Support deploying and using external private registry packages

[ericallam]

Currently we can support private registry NPM packages that are bundled, but we cannot support any that are external (and thus installed with npm i during the image build phase).

To support this we'll need to copy the .npmrc file before running npm i. It would be good if everything for this would be in a build extension that copies the local .npmrc file and adds an NPM_TOKEN build env var.

[yadavshubham01]

please Checkout this PR #1405

[yadavshubham01]

hey @ericallam can you please provide me more context about this issue