Allow dynamically and statically linked PTs to be loaded into a Tor binary
Currently PTs are always executed as subprocesses of Tor. This have its downsides on certain platforms such as iOS where we cannot have actual subprocesses and people have to do various hacks for them to work well on the platform.
It would be useful if we had a mechanism for loading PTs as plug-ins of the Tor process (via .so's and .dll's), but also to allow developers to statically link in the PT into the Tor binary to save space.