Psiphon in probe-engine

[bassosimone]

The Psiphon test is fully integrated inside of probe-engine making it ready for deployment.

The steps required are:

• [x] httpx implement NewClientWithProxy (ooni/netx#130)
• [x] HTTPDoConfig: add support for proxy function (ooni/netx#131)
• [x] porcelain: add proxy and byte counting support (ooni/netx#137)
• [x] build.sh: re-enable psiphon (ooni/probe-cli#79)
• [x] x/logger: mimic cURL logging (ooni/netx#137)
• [x] psiphon: use netx so we fill the HTTP template (ooni/probe-engine#129)
• [x] psiphon: make sure data format matches old implementation (ooni/probe-engine#128)
• [x] psiphon: make sure we're using latest version of dependencies (ooni/probe-engine#141)
• [x] orchestra: implement authenticated requests (ooni/probe-engine#148)
• [x] psiphon: fetch config via authenticated orchestra (ooni/probe-engine#109)
• [x] psiphon: use config from orchestra (ooni/probe-engine#167)
• [x] orchestra: MK compatible platform names (ooni/probe-engine#169)
• [x] orchestra: store login and tokens on persistent storage (ooni/probe-engine#164)
• [x] psiphon: follow-up of fetching config from orchestra (ooni/probe-engine#168)

[hellais]

@bassosimone can you add here a checklist of what are the next steps for getting this deployed?

I believe we now have the psiphon test inside of probe-engine, but building is disabled in probe-cli as we didn't want to include it for the stable release.

[bassosimone]

@hellais here's what I currently have sketched out (@bassosimone I moved the items list into the top issue ~ Arturo).

[bassosimone]

This is done!