Add rails-erd

MSP-9816
Metasploit::ERD.namespace_entities

MSP-9816

`Metasploit::ERD.namespace_entities` will find all
`ActiveRecord::Base.descendants` under a given namespace name.
Metasploit::ERD::Entity::Namespace

MSP-9816

Move Metasploit::ERD.namespace_entities to
Metasploit::ERD::Entity::Namespace#classes.