Add a way to proxy upstream requests instead of external links

[amvanbaren]

Fixes #452 Added UpstreamProxyService

Testing steps

• Add to dev/application.properties:

ovsx:
  upstream:
    proxy:
      enabled: true
    url: https://open-vsx.org/

• Run the server
• Test the upstream endpoints
• Expected result: all open-vsx.org links are rewritten to localhost:8080