0.24.0

Synced to flipcash2-protobuf-api@0e167fa6,
picking up #164. Only
chat.v1 moved.

This release adds pinned messages to chats. Nothing existing changed on the wire. The one
source break is a new oneof case, appended like every other addition here.

Breaking

• chat.v1.MetadataUpdate's kind oneof gains pins_changed (field 9). An exhaustive switch
  over the Swift oneof, or a when over Kotlin's KindCase without an else, needs a case for
  it before it compiles. A when with an else compiles but routes pin events into that
  branch, so check what it does.

Added

• chat.v1.Chat.PinMessage, UnpinMessage and GetPinnedMessages, with their request and
  response messages. PinMessageResponse.Result is OK, DENIED, NOT_FOUND,
  MESSAGE_NOT_FOUND, TOO_MANY_PINS, MESSAGE_NOT_ALLOWED (0–5); the other two results are
  OK, DENIED, NOT_FOUND (0–2). Pin and unpin are no-ops returning OK when nothing would
  change, and both return the chat's PinSummary on OK. The server caps a chat's pins (5
  today) and may raise the cap, so clients must not hardcode it.
• chat.v1.Pin (message_id, pinned_by, pinned_at, message) and chat.v1.PinSummary
  (latest, count, version). version moves by one on every real pin or unpin and is
  compared like RosterSummary.version: keep the greater.
• chat.v1.Metadata.pin_summary (field 19). It may be unset on feed RPCs such as
  GetDmChatFeed and GetGroupChatFeed, and an unset value there must not clear a summary the
  client already holds.
• chat.v1.MetadataUpdate.PinsChanged, with pinned or unpinned and the summary after the
  change.
• chat.v1.ViewerState.Permissions.can_pin (field 4). Today it is true for either member of a
  DM and for a group's creator while they are a member.

Unchanged

Nothing was renumbered. No service, RPC, field or enum case was removed or changed type.