Make graphql id fields globally unique

The GraphQL::ID_TYPE we use in GraphQL should be globally unique, but right now we're just dropping in the database id.

I think we should start being consistent, and build a real, globally unique id we can decode and use to find a single object (preferably using the BatchModelLoader).

This will also allow us write cleaner mutations that require an ID like that to load their object: https://graphql-ruby.org/mutations/mutation_classes.html#auto-loading-arguments

Since all of our types are now a Types::BaseObject, I think we could introduce an id method, that would take precedence over the PresentedClass#id method.

Roughly like this:

app/graphql/types/base_object.rb:

def id
  GitlabSchema.id_from_object(object)
end

app/graphql/gitlab_schema.rb:

def self.id_from_object(object)
  Base64.encode("#{object.class.name}-#{object.id}"
end

def self.object_from_id(object_id, _context)
  class_name, id = Base64.decode(object_id).split("-")
  return unless class_name.present? && id.present?

  Gitlab::Graphql::Loaders::BatchModelLoader.new(class_name.constantize, id).find
rescue NameError 
  raise Gitlab::Graphql::InvalidId, "Invalid Graphql ID"
end

We'll need to double check any fields that already take an ID_TYPE argument.