Add project_templates_during_registration experiment FF

What does this MR do and why?

We believe that in order for a new signup to recognize value they need to start using the product as soon as possible. Before value can be recognized, a project must be setup which can take time and effort.

To verify that, we will test displaying common project templates during registration and gauge engagement with them.

This is the first part with the backend changes needed for the new UI dropdown: #441981 (closed)

Also this includes future feature flag.

MR acceptance checklist

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Related to #441981 (closed)