Reporting identity information back to Flagsmith API when using local evaluation

[matthewelwell]

When using local evaluation, we want to be able to see the identities and their traits that have been evaluated locally.

As part of this, we want to be able to create a read-only view of the identities page since identity overrides won't work still.

[dabeeeenster]

Keep a hashmap of identities and traits in the SDK when running in local eval mode and send all the Identity strings up to the API every N seconds. Keep a record of which Identities we have sent so they only get sent once. Have a configurable cache size in the SDK so LRU (maybe?!) entries are evicted. This will result in resends but it is unavoidable unless we stop caring about SDK memory consumption.