Add support for player metadata

[kirre-bylund]

This contains two ugly workarounds mentioned below, but has now been tested and works.
Workarounds:

• Backend needs enum values lowercased, so the request body for metadata operations converts the operations list to a new internal type that has the enums as strings and forces them to be lowercase. This is memory and cycle waste but is unfortunately how it needs to be at the moment.
• When operations are performed to self, then the source field and source id field need to be either dropped or set to something valid. So I opted for setting it to the current player ULID.
  • This is no longer true. I changed it to setting source and source_id to null which worked fine. That is a simpler solution.