Arti: No pluggable transports, unsafe as enabled default

Direct connections to the tor network are trivially detected, by even consumer-grade DPI systems (see #1347). In countries that use deep packet inspection such as China and Iran, directly connecting to the tor network by default could put users in danger. This is the primary reason why bridges and pluggable transports exist.

As mentioned in docs/TOR-INTEGRATION.md, vendored Arti is not built with bridges support despite upstream supporting bridges and pluggables.

Unless there's some severe issue with Arti's implementation of pluggables, this is a pretty glaring safety issue that is straightforward to resolve. If Arti can't use pluggables on iOS for whatever reason, tor routing should really be disabled by default.