listen to RPK change event and automatically reload

[mistafunk]

continuing #58

register file monitors for active RPKs and invalidate their cache entries if they change on disk, eventually allowing automatic recook as well (maybe controllable with a checkbox on pldAssign)

useful libraries for this:

• HDK itself has file monitor functionality (see the FS module)
• https://pocoproject.org/docs/Poco.DirectoryWatcher.html (impeded by conan recipe not supporting -fPIC)

[mistafunk]

(internal ticket CE-6317)

[mistafunk]

try very hard to use the file monitor callbacks by Houdini itself before adding new deps