Sender Data: Support sender data in InboundGroupSession and InboundGroupSessionPickle

[andybalaam]

As part of supporting Invisible Crypto, we want to store information about the sender with an InboundGroupSession (and persist it, so it needs to be in InboundGroupSessionPickle too).

Part of #3544 which is part of Invisible Crypto.

Add something like this to InboundGroupSession and PickledInboundGroupSession

    sender_data: InboundGroupSessionSenderData,

Where

/// Information on the device and user that sent the megolm session data to us
#[derive(Clone)]
pub enum InboundGroupSessionSenderData {
   /// We have not yet found the (signed) device info for the sending device
   UnknownDevice {
      // we may need to handle unsigned and unknown devices separately, which
      // probably necessitates a flag here
      retry_details: RetryDetails,
      legacy_session: bool,
   },

   /// We have the signed device info for the sending device, but not yet the
   /// cross-signing key that it was signed with.
   DeviceInfo { 
       device_keys: DeviceKeys, 
       retry_details: RetryDetails,
       legacy_session: bool
   },

   /// We have figured out the MSK and user
   SenderKnown {
       mxid: OwnedUserId,
       msk: Ed25519PublicKey,
       /// Whether, at the time we checked the signature on the device,
       /// we had actively verified that `msk` belongs to the user.
       msk_verified: bool,  // or maybe VerificationState
   },
}

#[derive(Clone)]
struct RetryDetails {
   retry_count: u8,
   next_retry_time_ms: MillisecondsSinceUnixEpoch,
}

PickledInboundGroupSession must be deserializable from an existing stored inbound group session (ie, one which lacks a sender_data field). In this case, it is a “legacy” session; we set it to UnknownDevice with legacy_session=true and a next_retry_time_ms in the near future).