Draft: Support Snowflake bridges

• Design UX. At the moment the ux is very bad: the user needs to type snowflake 127.0.0.1:1 (or any other IP) for it to work. There is no explaination about why this works.
• Testing; that's not trivial
  • testing with the real Tor is a bit easier. Status: almost there.
    • make it work at least if the user explicitly runs the test suite with --disable-chutney
    • make it work using the real Tor without passing other options. Status: WIP
  • mocking this with chutney might be difficult: to mock snowflake we would need to mock both some bridges AND a broker. This would probably also require us to have a complex TLS configuration.

Closes #5494