Expose an API to inspect room key request replies

[BillCarsonFr]

Decription

This API would allow clients to query for sent key requests, and see the replies by other devices to this request. This API would be used by clients:

• to display feedback on Unable To Decrypt errors (like there are pending request for this keys, this device refused to share because of this withheld code...)
• expose some devtools
• Help in the creation of integration tests

   ┌────────────────────────────┐               ┌─────────────────────────────────────────────────┐
   │     OutgoingKeyRequest     │               │      RequestReply                               │
   │                            │               │                                                 │
   │                            │               │  - fromUser: string                             │
   │                            │  replies    * │                                                 │
   │   - requestId              ├───────────────►  - fromDevice: String?                          │
   │                            │               │                                                 │
   │   - roomId                 │               │  - result: Success(index) | Failure(withheld?)  │
   │                            │               │                                                 │
   │   - sessionId              │               │                                                 │
   │                            │               └─────────────────────────────────────────────────┘
   │   - algorithm              │
   │                            │
   │   - state                  │
   │                            │
   │                            │
   │                            │
   └────────────────────────────┘

The SDK could keep a one week history of outgoing key request.