[rfq]: add validation to AddAssetBuyOrder and AddAssetSellOrder RPCs

[guggero]

Fixes #1053.

Adds basic validation to AddAssetBuyOrder and AddAssetSellOrder RPCs: We want to have a channel with the given asset to the given peer when adding an order. Otherwise RFQ negotiation doesn't make sense.

We add an exception for our integration tests, where we can't create full-blown asset channels (in the tapd standalone integration tests). A new flag can be specified that then loosens the requirement to just a normal, active channel with the peer.

[coveralls]

Pull Request Test Coverage Report for Build 11955125756

Details

• 0 of 59 (0.0%) changed or added relevant lines in 1 file are covered.
• 10 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-0.006%) to 41.23%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
rpcserver.go	0	59	0.0%

Files with Coverage Reduction	New Missed Lines	%
tappsbt/create.go	2	53.22%
commitment/tap.go	2	84.17%
tapchannel/aux_leaf_signer.go	3	36.33%
tapgarden/caretaker.go	3	68.59%

Totals
Change from base Build 11953454712:	-0.006%
Covered Lines:	25552
Relevant Lines:	61975

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💛 - Coveralls

[ffranr]

I think we should consider dropping the term "balance" from the new req arg name.

We can just call it SkipAssetChannelCheck and the "channel check" can include checking for an existing channel and any balance check that we may add later (is absent from this PR i think?).

[guggero]

@ffranr sorry for the premature request for review before. Everything should be green now (minus flakes) and comments addressed.

[gijswijs]

tACK 🎉

Just one nit.

I would like to see a future where tapd can be tested with some sort of a lnd-mock so that we can mimick proper asset channels, and not need to depend on flags like this.