fix(engine): break circular dep betwen session and tunnel (#295)

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.

internal/engine/session_nopsiphon.go

@@ -2,7 +2,10 @@
 
 package engine
 
-import "context"
+import (
+	"context"
+	"errors"
+)
 
 // FetchPsiphonConfig fetches psiphon config from the API.
 func (s *Session) FetchPsiphonConfig(ctx context.Context) ([]byte, error) {
@@ -12,3 +15,12 @@ func (s *Session) FetchPsiphonConfig(ctx context.Context) ([]byte, error) {
 	}
 	return clnt.FetchPsiphonConfig(ctx)
 }
+
+// sessionTunnelEarlySession is the early session that we pass
+// to tunnel.Start to fetch the Psiphon configuration.
+type sessionTunnelEarlySession struct{}
+
+// FetchPsiphonConfig implements tunnel.Session.FetchPsiphonConfig.
+func (s *sessionTunnelEarlySession) FetchPsiphonConfig(ctx context.Context) ([]byte, error) {
+	return nil, errors.New("no embedded configuration file")
+}