Migrate subscription metadata when subscriptions are migrated to MV3

Background

Currently, we migrate subscriptions by removing the old MV2 subscription and adding an equivalent MV3 subscription. However, this clears out any metadata associated with the subscription that might be used by the data team for user counting.

Use case

downloadCount and version are important metadata used in user counting. subscription.version is sent on the update request as subscription.lastVersion. Maintaining this metadata allows the data team to identify that this is an existing user who has migrated to MV3, and not a new MV3 install.

What to change

When we migrate subscriptions, copy any relevant metadata from the old subscription to the new subscription. Specifically, make sure that downloadCount and lastVersion are migrated.