tapchannel: add new IsCustomHTLC method to aux traffic shaper

[Roasbeef]

This lets other sub-systems determine if an HTLC is for a custom channel, without needing to know about the precise record types.

[coveralls]

Pull Request Test Coverage Report for Build 13001912377

Details

• 0 of 3 (0.0%) changed or added relevant lines in 1 file are covered.
• 29 unchanged lines in 6 files lost coverage.
• Overall coverage decreased (-0.009%) to 40.705%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tapchannel/aux_traffic_shaper.go	0	3	0.0%

Files with Coverage Reduction	New Missed Lines	%
tappsbt/create.go	2	53.22%
fn/option.go	3	43.3%
tapgarden/caretaker.go	4	68.11%
asset/asset.go	4	76.82%
asset/mock.go	6	91.98%
universe/interface.go	10	52.81%

Totals
Change from base Build 13001676154:	-0.009%
Covered Lines:	26748
Relevant Lines:	65712

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💛 - Coveralls

[GeorgeTsagk]

Lgtm!