add support for npm adduser/login command

Configuring credentials is very often required to publish to and get packages from Nexus npm repos.

Our current process is to ask the end user to manually base64 encode their credentials and insert an _auth property into their .npmrc file with the value.

base64 encoding credentials manually is done differently on every OS and can lead to 401 authentication errors if not done correctly.

Recent npm cli versions also fail with authentication errors when _auth is correctly set, but the "email" property is not set, even though Nexus does not need this property itself.

The npm adduser command handles all these manual steps and sets up the .npmrc file correctly. It is something an npm user is used to using and is documented at the npm site. Nexus should support the npm adduser command, to make adding auth information to .npmrc easier and make getting started with Nexus npm repos quicker.

https://docs.npmjs.com/cli/adduser