Support custom ports

[visola]

User Story

As a user, I want to be able to run the proxy and admin server in custom ports.

Description

It would be good to have a way to customize the ports for the server and admin UI to help with local conflicts.
Ideally, support environment variables and command line arguments.

Acceptance Criteria

• Ports can be configured through environment variables through:
  • GO_PROXY_ADMIN_PORT - port to bind admin UI to
  • GO_PROXY_PORT - port to bind the proxy to
• Proxy server uses set port and shows port correctly when started
• Admin UI uses correct port and shows port correctly when started
• Admin UI link to server points to correct port