[feature]: multi RFQ send: show how much of an asset was sent in each shard

[ZZiigguurraatt]

When making a multi RFQ send with #1613, the payment can be sent in multiple shards through different channel peers that have different rates. SendPayment streams the quotes that were received before trying to make a payment, but we don't have an easy way to know what quotes were actually used and how much assets were sent using each quote.

We need to know how much asset was sent in each shard and what that rate was for that shard. It would be also nice to summarize the total assets sent and then compute a weighted average rate for the asset through all shards.