Multi rfq send itest

[GeorgeTsagk]

Description

Enhances the multi-rfq itest to cover multi-rfq send functionality. In the following topology

	  /---[sats]--> Erin --[assets]--\
	 /                                \
	/                                  \

 Charlie  -----[sats]-->  Dave  --[assets]---->Fabia

	\                                  /
	 \                                /
	  \---[sats]--> Yara --[assets]--/

we now also get Fabia to pay Charlie back, with amounts that exceed the capacity of each individual channel. We also use a hold invoice to validate that multiple HTLCs were added in different channels.

Related PRs (in order of merge sequence):
lightningnetwork/lnd#9980
lightninglabs/taproot-assets#1613
lightninglabs/loop#968

[ZZiigguurraatt]

Seems like you should update

https://github.com/GeorgeTsagk/lightning-terminal/blob/c7dbe80f1b6463bb01fe2f94f72cf52b6b6ab9de/itest/litd_custom_channels_test.go#L2973-L2977

to mention the send test, possibly renaming the function name?

[ZZiigguurraatt]

In the following topology

	  /---[sats]--> Erin --[assets]--\
	 /                                \
	/                                  \

 Charlie  -----[sats]-->  Dave  --[assets]---->Fabia

	\                                  /
	 \                                /
	  \---[sats]--> Yara --[assets]--/

Do we see any value in adding tests where Charlie has more or less channels than Fabia? Like this route seems a bit too simple because both sender and receiver kind of need the same sharding.

I'm also wondering if we should test where Charlie-Dave have asset channels and not sats channels.

I don't think this test should be removed, but I think there should be some more complex network that tests more than what this test can do in another function.