Support for npm private registry

[loispostula]

Fix 1012

Implement support of private npm registry

Test

To test this pull request, you can run the following commands:

cp pkg/plugins/resources/npm
go test

Additional Information

When working with a private registry, it is necessary to specify the registry URL and an authorization token. These details can be provided in the resource specification and will be used as the default registry for connecting.

The resource code also checks for the presence of an npmrc file. This file contains information about scoped packages and can be specified in the spec.NpmrcPath field or defaults to $HOME/.npmrc.

The code will first establish the default registry by using the provided URL and token or defaulting to the npm registry. It then parses the npmrc file to identify additional registries and determine which scoped packages should use them.

When searching for a package, the resource code checks if it is a scoped package and if the scope has a custom registry configured. If so, it uses that registry, otherwise, it uses the default registry.

I've also fleshed out the test to introduced a mocked registry client

Tradeoff

Potential improvement

[olblak]

I don't have access to a private NPM registry so I can't really test it but the code part looks great.
Thanks for the pullrequest.