[Protocol] Add room chat channel extensions and catalog payload (#7475)

* [Protocol] Add room chat channel extensions and catalog payload

* [Protocol] Add per-channel write level and banner to room channel catalog

Adds two optional fields to ServerInfo_RoomChannel:

- write_access_level: minimum privilege level required to send to the
  channel. When unset, writing requires the same level as reading
  (access_level), preserving the previous single-level behavior. This
  allows read-mostly channels such as a developer news feed: readable
  by everyone, writable only by developers.
- banner_message: static notice shown at the top of the channel,
  analogous to the room join message on Main. Empty means none.

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Co-authored-by: Lukas Brübach <Bruebach.Lukas@bdosecurity.de>

libcockatrice_protocol/libcockatrice/protocol/pb/serverinfo_chat_message.proto

@@ -13,4 +13,5 @@ message ServerInfo_ChatMessage {
     optional string target_type = 6; // target type (room,game,chat)
     optional string target_id = 7;   // id of target
     optional string target_name = 8; // name of target
+    optional string channel = 9;     // channel id, for room_channel targets
 }