This diff breaks the circular dependency between session and
tunnel, by introducing the concept of early session.
An early session is a session that is able to fetch the psiphon
configuration file _only_ if it's embedded in the binary.
This breaks `miniooni --tunnel=psiphon` for users who have
access to the OONI backend. They are not the users we are
writing this feature for, though, so I think this is reasonable.
At the same time, this opens up the possibility of creating
a psiphon tunnel when constructing a session, which is the
approach I was following in https://github.com/ooni/probe-cli/pull/286.
This work is part of https://github.com/ooni/probe/issues/985.
Once this diff is in, I can land https://github.com/ooni/probe-cli/pull/286.
We use an optional build tag to hide this configuration. When you
choose this configuration, you need to provide the encrypted config
as well as the corresponding decryption key.
This is not the final design. This is an interim design to start
working and experimenting with this functionality. The general
idea here is to support psiphon in the binaries we build without
committing the psiphon config to the repository itself.
Part of https://github.com/ooni/probe/issues/985
