Add support for non-PK columns

[tylerwillingham]

• Implements Support for using GlobalID with configurable columns #184

With this proposal, GlobalID::Identification would implement global_id_column to set the desired column for GlobalID to reference on a per-model basis

i.e.

class Person
  attr_accessor :id, :external_id

  global_id_column :external_id
end

Person.new(id: "id-value", external_id: "external-id-value").to_gid.to_s
# => gid://app/Person/external-id-value

I think this is particularly useful for teams in the process of migrating off of auto-incremented primary keys, or are interested in using unsigned GIDs with some level of obscurity that's not attainable without starting to make a bigger change like actually implementing UUIDs as PKs

[tylerwillingham]

A question I'm anticipating and open to solving is how do actually find the associated record?

I'm not sure how much we want to couple the solution inside of globalid to ActiveRecord. As-is, this would work in one of my applications because we use the friendly_id gem. I think this is easily achievable with where and find_by! but I don't know how agnostic from ActiveRecord this gem is intended to be