Extend replicants via arbitrary Ruby code

MSP-11673

* Implements a #register_extensions method on Msf::Module
* Any registered Ruby modules will extend the cloned module returned by #replicant
Just use constants directly

MSP-11673
Land #4298, arbitrary Ruby extension for replicant

MSP-11673

* Adds Msf::Module#register_extensions
* Extensions are arbitrary Ruby modules
* Allows overriding of psuedo callbacks