Add convar for spectate order

[anf3is]

This PR aims to support custom spectating order (for example: Jaeger → Wildcat → Vegas → [other/fallback]).
The configured order should be respected when cycling between marines while preserving the current behavior that all alive marines remain valid spectate targets.

WIP Info

• SpectateNextMarine is server-side. Because of that we need to send client's spectate order to server.
• newmapsettings.cfg is too early for rd_set_spectate_order 😭
• rd_set_spectate_order operates on CASW_Player. Do we need to call rd_set_spectate_order after client is fully connected and CASW_Player for this client is created?

Tasks

• Add client-side rd_spectate_order for storing client's preferences
• Find a place to automatically call rd_set_spectate_order if rd_spectate_order is set
• Mark rd_set_spectate_order as hidden