Proxy-based-servers: Add notification option when unmarked player joins

[atomy]

the new player notification may not be always sufficient cause players may leave shortly after joining and may rejoin later then there'll be no "New player" notification anymore even though they havent been checked by an admin

[atomy]

🤖 Dispatched to the backend and app repos:

https://github.com/atomy/rustdroid-proxy/issues/11
https://github.com/atomy/rustdroid-android/issues/39

[atomy]

🤖 All sub-project tickets are done: https://github.com/atomy/rustdroid-proxy/issues/11 and https://github.com/atomy/rustdroid-android/issues/39.