[PACE-76] Change Data Policy primary key

[trietsch]

A Data Policy currently is unique based on the id and the platform_id (and version), but this is only enforced in the database.

To ensure consistency across the API and database, @bvdeenen and @trietsch propose the following change:

• Remove platform_id from the database migration in the data policy primary key
• A Data Policy will be unique on the id and the version
• If the version is omitted, the latest version is assumed
• In RPCs, id and version should be included
• A Data Policy reference message (including id and version) or a URN should be introduced to easily refer to a single unique Data Policy.

_{From SyncLinear.com | PACE-76}