wip: support rails mattr_accessor variants

[zzak]

This is just an early proof-of-concept, but the idea is that we can hold a reference to the following rails extensive attributes API so they can be displayed and searchable from api.ror.org.

• mattr_*
• cattr_*
• thread_mattr_accessor
• thread_cattr_accessor

This should be just a lot of copy and paste, and adding tests, happy to hand this off if anyone is interested!