Pass optional record in blob finder methods

[gmcgibbon]

Summary

Allow record to be optionally passed to blob finders to make sharding easier.

To generically support sharded Active Storage models, we need to specify a model to shard attachments by. The polymorphic record can then be used by a developer in an overridden method:

ActiveSupport.on_load(:active_storage_blob) do
  include ShardedBlob
end

module ShardedBlob
  extend ActiveSupport::Concern
  included do
    def self.find_signed(id, record:)
      find_by!(id: ActiveStorage.verifier.verify(id, purpose: :blob_id), shard_id: record.shard_id)
    end
    # ...
  end
end

This change can be misconstrued as a mistake as the framework doesn't use the record parameter by default, so I've added some tests to make sure it doesn't get removed by mistake.

cc @georgeclaghorn @rafaelfranca