[SEP-41] Allow transfer from muxed addresses

[hugo-heer]

Right now with the token contract you can transfer to a Muxed address but you can't transfer from a Muxed address. This is a problem if we want to use the muxed addresses for wallet custody since we can't observe which id is emitting the transfer.

[dmkozh]

Thank you for starting this discussion.

The muxed destination has been added to the transfers as the minimal support for the known use case (deposit deposit to multiplexed destinations has a lot of use cases). At the time when this has been added, there was no strong support for the muxed source use case. This is definitely not out of the question, but it would be nice to understand the use case better in order to dedicate an effort to this.

In the case of muxed destinations there is no real alternative to the current approach, as anyone can make a transfer to the custodial account and the account owner needs a generic way for processing all the incoming transfers, which can be achieved by ingesting the events with the respective muxed destination. But in case of the transfer source, the account owner must take care of building the transaction and updating the 'virtual' balances, which allows for alternatives. For example, the custodial balance can be updated on transaction success. This may not be necessarily optimal, but monitoring the transaction success seems like something a wallet would want to implement anyways? Basically, there is much more control over outgoing transfers, which is why it would be great to understand the benefits that muxed sources would provide vs the added implementation and protocol complexity cost.