0.21.0

Synced to flipcash2-protobuf-api@b77bf78f,
picking up #151,
#152 (docs only),
#154 (docs only),
#155 and
#156. paymentrequest.v1 is
new. activity.v1, chat.v1, intent.v1 and messaging.v1 moved.

This release adds payment requests (group splits), payments to several group members at once, and
moderation RPCs for group creators. Upgrading breaks source, not the wire. CashContent renamed
two fields and moved one into a oneof, so code reading amount or receiver stops compiling. Two
enums gained a case, appended at the end, so no existing value moved.

Breaking

• messaging.v1.CashContent.amount (field 2) is now total_amount. Same type and number. For a
  payment to several people it is the whole payment.
• messaging.v1.CashContent.receiver (field 3) is now single_receiver, inside a new
  oneof recipient. Kotlin's getReceiver()/hasReceiver() become getSingleReceiver()/
  hasSingleReceiver(); Swift's receiver becomes singleReceiver. An older DM message that
  predates per-message receivers still has neither member of the oneof set.

The binary encoding of both fields is unchanged, so messages written before and after the rename
decode the same way.

Added

• paymentrequest.v1.PaymentRequest, a new service with five RPCs: CreatePaymentRequest,
  GetPaymentRequest, GetChatPaymentRequests, ClosePaymentRequest and MarkPaid. The one kind
  today is a split in a group chat (GroupChatPaymentRequest with a Split of per-member
  Shares). DMs are not supported. No event announces a change to a request; clients refetch it.
  • In a private group the split's title and description are EncryptedPaymentRequestText under
    the chat key. Everything else, including shares and payments, is visible to the server.
  • GetPaymentRequestResponse.payment_intent_id is the intent ID a debtor's payment of their share
    must use. The server issues one per share, so a share is paid at most once.
• messaging.v1.PaymentRequestWidget, sent as WidgetContent.payment_request (oneof field 3). It
  carries only the request's ID.
• intent.v1.ChatMetadata.requested_payment (oneof field 6), a RequestedPayment naming the
  payment_request_id whose share a SendPublicPayment pays.
• messaging.v1.CashContent.multi_receiver (oneof field 5), a Payments list of 2 to 10
  Payment { receiver, native_amount, quarks }. A group payment to several members is an ocp
  SendPublicMultiPayment.
• messaging.v1.CashContent.payment_request_id (field 6), set only on the cash message posted when
  a debtor pays their share.
• Chat moderation RPCs on chat.v1.Chat: SilenceUser, UnsilenceUser, BanUser, UnbanUser and
  GetRestrictedUsers, with SilenceState, BanState and RestrictedUser. The chat's creator
  cannot be silenced or banned.
• chat.v1.ViewerState.restrictions (field 3), carrying the viewer's silence. A timed silence
  lapses with no event, so the client owns the countdown.
• chat.v1.ViewerState.Permissions.moderation (field 2), a ModerationPermissions with
  can_silence and can_ban.
• JoinChatResponse.Result.BANNED = 4 and EnterLobbyResponse.Result.BANNED = 6, both appended.
  An exhaustive switch or when over either needs the new case before it compiles.

Changed

• Reaction boosts are now allowed in DMs. A DM boost pays the other member the whole amount as one
  SendPublicPayment. No field changed; the activity.v1.BoostedReaction and
  intent.v1.ChatMetadata.ReactionBoost docs now cover DMs.
• SendMessage and EditMessage document that ENCRYPTION_REQUIRED also applies to DMs.

Unchanged

Nothing was renumbered. No service, RPC or enum case was removed. Apart from the CashContent
rename above, no field was removed or changed type.