Shielding transfer over IBC supported by Hermes

[yito88]

Currently, for shielding transfer from another chain to Namada, a user has to do a 2-step operation.

1. ibc-gen-shielded command to generate the MASP proofs on the target Namada
2. IBC transfer with the proofs in the memo field of ICS-20 packet data

Like regular IBC transfers, requesting IBC transfers without setting the MASP proofs is helpful to senders. (The difference is only specifying a payment address as a receiver.)

1. Send tokens with a payment address as a receiver from the source Namada like regular IBC transfers (ibc-transfer)
2. Hermes(IBC relayer) checks the transfer event
3. Hermes requests the MASP proof generation to the target Namada when the receiver address is a payment address
4. Hermes submits a transaction with MsgRecvPacket and the proof as transaction data
   • Like sending tokens: Namada's MsgTransfer has Option<IbcShieldedTransfer> (proof)
     • It means that we don't use the memo field anymore
5. Target Namada handles the message and the proof