Adding Secure Files object storage migration rake task

What does this MR do and why?

Adds a rake task to migrate Secure Files from local storage to object storage.

Closes gitlab-org/incubation-engineering/mobile-devops/readme#125 (closed)

How to set up and validate locally

1. Ensure Secure Files object storage is disabled via gdk.yml

   object_store:
     enabled: true

2. Follow the instructions in the docs to upload one or more Secure Files to a project.

3. Create the ci-secure-files bucket in your local object storage (http://127.0.0.1:9000 if using minio)

4. Reconfigure GDK to use object storage for Secure Files changing gdk.yml, then run gdk reconfigure; gdk restart.

   object_store:
     enabled: true

5. Run the rake task bundle exec rake gitlab:ci_secure_files:migrate

6. The output should be similar to this example:

I, [2023-06-05T11:28:11.577904 #39705]  INFO -- : Starting transfer of Secure Files to object storage
I, [2023-06-05T11:28:11.617101 #39705]  INFO -- : Transferred Secure File ID 68 (upload-keystore.jks) to object storage
I, [2023-06-05T11:28:11.620536 #39705]  INFO -- : Transferred Secure File ID 69 (sample.p12) to object storage

MR acceptance checklist

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