* [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>
