Subscriptions: add support for imports to paid plans

### Impacted plugin

Jetpack

### What

paid subscribers import

### How

At the moment, when a user is migrating to WordPress.com and they have a paid subscription they need to request the subscribers to re-subscribe, even if it was a Stripe payment. Recently, I had several requests like that. If they have just several subscribers it is not a big issue but if there are many it can be a blocker for a user.

The issue describes a Jetpack migration case where paid subscribers should be imported, but it names no files, tests, or entry points. First clarify the existing subscription import flow and the expected behavior for Stripe-paid subscribers; done should mean paid subscribers are imported without requiring them to re-subscribe.

Written by the indexing model from the issue text.