feat(dav): add a room building name to IRoomMetadata

Room backends can publish a building address but not a building name.
Clients that group rooms by building therefore derive one from the
address, in practice by taking the segment before the first comma. That
only works for backends that put the building name first: a room whose
address is "1098 XG, Amsterdam" ends up grouped under a postal code.

Add BUILDING_NAME so backends that hold the building name as a separate
field can publish it, and clients can stop guessing. The key is
optional, and nothing changes for backends that do not provide it.

Assisted-by: ClaudeCode:claude-opus-5
Signed-off-by: Rikdekker <Rikdekker@users.noreply.github.com>

Author: Rikdekker

lib/public/Calendar/Room/IRoomMetadata.php

@@ -37,6 +37,17 @@ interface IRoomMetadata {
 	 */
 	public const CAPACITY = '{http://nextcloud.com/ns}room-seating-capacity';
 
+	/**
+	 * The name of the building this room is located in
+	 *
+	 * Clients group rooms by building. Without this key they have to guess a
+	 * building from BUILDING_ADDRESS, which only works for backends that
+	 * happen to put the building name first.
+	 *
+	 * @since 35.0.0
+	 */
+	public const BUILDING_NAME = '{http://nextcloud.com/ns}room-building-name';
+
 	/**
 	 * The physical address of the building this room is located in
 	 *