Metasploit::Model::Module::Handler

[#56005352]

Move handler_type and new general_handler_type into new
Metasploit::Model::Module::Handler to mimic how handlers are used in
metasploit-framework more closely.  'Metasploit::Model::Module::Handler'
is a shared example that both verifies the correctness of the
self.handler_module in the Metasploit::Model::Module::Ancestor#contents
and can be used to verify the same interface in Msf::Handler::* Modules.