Upsert KVStore data

Upsert KVStore data for the specified keys and from the specified stores. If a World ID or Launch Context ID header are supplied, the respective Project ID header must also be supplied.

Any existing data under the given keys will be overwritten.

Data can be read from either the client or server store by both the user and Unreal server or service accounts. Client store data can only be written by the user who's store it is but can be read by the user and an Unreal server or service account. The server store can only be written by the Unreal server or service account but may be read by both the user and the server.

We offer a canonical place to store the username and avatar URL of each user. These can be found in the client store, at organization level, under the keys "_morpheus_username" and "_morpheus_avatar_url" respectively. The value written to the username reserved key will automatically be checked against the organization's username rules.