http proxy support for dapp-runner

[shadeofblue]

support for Local HTTP Proxy in dapp runner

independently from the descriptor -> just add local http proxy support to all dapps

• each HTTP proxy should specify the external and internal ports
• the external port can be provided as a range

[johny-b]

1. This should cover a case when we have more than one service
2. --> They should work either on separate ports
3. --> Important: the port should be passed to the data stream
4. Also: we should somehow detect "free" ports, as there might be more that one runner at a given time

[johny-b]

Details:

1. We don't care about anything related to http-proxy and/or network in the descriptor
2. BUT: when reading the descriptor, we have a mocked element that for each service:
   • sets some internal port (hardcoded value)
   • sets a range of external ports (also hardcoded)
3. Runner creates a network with hardcoded arguments, this will be improved in #18
4. Runner, for each service with specified (internal_port, range_of_external_ports):
   • finds a free port in range, or raises an exception (TODO: what exception?)
   • sets up the http proxy between (internal_port, this_found_free_port)
   • information about the selected free port is passed to the data stream. Preferably: as a complete base URL.