v10.1.0

• Added: the SDK names Relay protocol version 1 on HTTP. Each HTTP
  request to a Relay route carries the header
  x-open-e2ee-relay-protocol: 1. The fetch of the connection document
  carries no version. The mailbox socket still offers open-e2ee-relay.v1.
  The Relay reads a request without the header as version 1, so a Relay
  that serves version 1 serves this SDK and SDK 10.0.0.
• Added: HostedRelayUpgradeRequiredError. An HTTP call throws it when
  the Relay no longer serves the protocol version of the SDK. Its code is
  UPGRADE_REQUIRED, and its retryable is false. The SDK does not retry
  it. The class is exported from the main entry and from /client.
• Added: the connection reason upgrade-required. The mailbox
  subscription moves to stopped with this reason when the Relay closes
  the socket with code 4426 and the reason UPGRADE_REQUIRED. It does not
  reconnect and does not pull. A close 4400 with the reason
  PROTOCOL_UNSUPPORTED, and the HTTP code PROTOCOL_UNSUPPORTED, stay
  retryable.