UpdateBroadcastInput
UpdateBroadcastInputFields
The title recipients see in the notification. Pass null to clear it; a broadcast with none cannot be sent.
Set alongside contentFormat whenever the encoding of the new body differs from the stored one.
Which kind of sender senderUserId describes. Validated against the state the broadcast will
hold, so either half can be set on its own as long as the result is coherent: PLAIN_USER
needs a senderUserId, PLAIN must not have one.
Pass a target to replace the current one, or an empty channels list to clear it. Omit to leave
it unchanged. Clearing the target — or the sender — of a broadcast that is already scheduled is
rejected, because a scheduled broadcast has to stay sendable.

