This adds a StorageInterface that deals with the events storage. Two functions are defined: the saving and loading of events. Deleting events from the storage is intentionally not defined, as the individual providers are likely to have different mechanism for deletion and we do not think it will be required by libkazvclient.

CCBUG: https://iron.lily-is.land/T165

Type: add

Verify unit tests pass.

This patch should be reviewed together with a future patch that actually saves and loads data from the storage.