Add a way to include other optional services like pyreccaster

[kurup]

Add methods to facilitate including optional services such as pyreccaster. Such services would be disabled by default, which reduces the dependencies required by p4pillon.

[kurup]

My thinking is that this mostly impacts the Server class which would have a list of services that it initialises when the server is started and also there should be the option for each service to execute a method for each PV provided by the server (also when the server starts up).