Make it easier for mobile app developers to embed tor

The use-case that we are trying to improve is the following:

I am mobile app developer and I want to use Tor as part of my software and possibly instrument it in some way via it's control port.

See also: https://github.com/measurement-kit/measurement-kit/issues/86

The current situation of Tor integration on mobile platforms is a bit suboptimal. The best solution would allow developers to "execute" Tor in the same way on all platforms uniformly.

Currently the approaches taken for integrating Tor into mobile apps is one of two:

a) Tor is started as a process and the user speaks to it via the control port (this is only possible on Android though). Example: OrBot

b) The tor main() is wrapped and is called as part of some wrapper library written in the platform native language (this approach is the most common one on iOS). Example: Tor.Framework, CPAProxy

Both of these approaches are not the easiest from a mobile app developers perspective.

To make the developer experience easier, it would be best if some of the following things happened:

1. The build process of tor allowed you to also get a shared library

2. There is some API call or function that is documented and supported officially by the Tor library (1.) that allows the developer to start and stop tor.

If these two things were supported in tor natively, it would be possible to pack tor as a gradle and cocoapod dependency (which is the way in which mobile developers usually add dependencies to their project).

The other problem is related to usage of pluggable transports, which tor expects to be able to fork and start them and is something that we cannot do on iOS.

I suggest we can use this as a master ticket to track all the various improvements that are needed and attach child tickets for the various units of work that this can be split up into.