feat: add scope support loading plugins

[juanpicado]

Follow up of #1867
#3047

Close: #3047 #1867

Middleware

middlewares:
  audit:
    enabled: true
  '@xlts.dev/verdaccio-prometheus-middleware':
      metricsPath: /custom/path/metrics    

Auth

auth:
  '@scope/test':
    foo: 1
  htpasswd:
    file: ./htpasswd

etc etc ...

The error message would be like

 warn --- config file  - /home/xxx/.config/verdaccio/config.yaml
 error--- plugin not found. try npm install @scope/test
 fatal--- uncaught exception, please report this
Error: 
      @scope/test plugin not found. try "npm install @scope/test"