Metasploit::Model::Module::Ancestor :loading validation context

[#56005352]

Metasploit::Model::Module::Ancestor#handler_type can only be derived by
actually parsing the Metasploit::Model::Module::Ancestor#content, but
there needs to be a way to check that the
Metasploit::Model::Module::Ancestor is valid before parsing its content,
so add a :loading validation context that will disable the #handler_type
presence validation.