Allow custom purpose for ActiveStorage signed IDs

This commit allows passing a custom `purpose:` keyword argument to both
`find_signed!` and `signed_id`. This matches the signature of the
`super` methods in ActiveRecord, and it is already possible with the
non-bang [`find_signed`][find_signed] method, since that one doesn't
have an override in ActiveStorage (although perhaps it should, to set
the default :blob_id purpose)

This is useful in cases where you want to further isolate different
types of blobs, for example if they have different levels of
authorization.

[find_signed]: be11d1b6e8/activerecord/lib/active_record/signed_id.rb (L42-L48)

activestorage/app/models/active_storage/blob.rb

@@ -74,8 +74,8 @@ class ActiveStorage::Blob < ActiveStorage::Record
     # that was created ahead of the upload itself on form submission.
     #
     # The signed ID is also used to create stable URLs for the blob through the BlobsController.
-    def find_signed!(id, record: nil)
-      super(id, purpose: :blob_id)
+    def find_signed!(id, record: nil, purpose: :blob_id)
+      super(id, purpose: purpose)
     end
 
     def build_after_upload(io:, filename:, content_type: nil, metadata: nil, service_name: nil, identify: true, record: nil) #:nodoc:
@@ -143,8 +143,8 @@ class ActiveStorage::Blob < ActiveStorage::Record
   end
 
   # Returns a signed ID for this blob that's suitable for reference on the client-side without fear of tampering.
-  def signed_id
-    super(purpose: :blob_id)
+  def signed_id(purpose: :blob_id)
+    super
   end
 
   # Returns the key pointing to the file on the service that's associated with this blob. The key is the

activestorage/test/models/attachment_test.rb

@@ -91,6 +91,14 @@ class ActiveStorage::AttachmentTest < ActiveSupport::TestCase
     assert_equal blob, ActiveStorage::Blob.find_signed!(signed_id)
   end
 
+  test "getting a signed blob ID from an attachment with a custom purpose" do
+    blob = create_blob
+    @user.avatar.attach(blob)
+
+    signed_id = @user.avatar.signed_id(purpose: :custom_purpose)
+    assert_equal blob, ActiveStorage::Blob.find_signed!(signed_id, purpose: :custom_purpose)
+  end
+
   test "signed blob ID backwards compatibility" do
     blob = create_blob
     @user.avatar.attach(blob)