[wallet 3/3]: group key support for channel funding

[guggero]

Adds the ability to fund multiple tranches of a grouped asset into a channel.

TODO:

• Update channel closer code
• Update sweeper code
• Extend itest with coop and force close test cases

Corresponding itest: lightninglabs/lightning-terminal#987

Part of the group key support saga:

• Part 1: [wallet 1/3]: refactor in preparation for group key channel funding #1402
• Part 2: [wallet 2/3]: refactor funding logic to use allocation code #1407
• Part 3: This PR

[coveralls]

Pull Request Test Coverage Report for Build 13547766726

Details

• 14 of 91 (15.38%) changed or added relevant lines in 6 files are covered.
• 74 unchanged lines in 14 files lost coverage.
• Overall coverage decreased (-0.05%) to 54.522%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
rpcserver.go	0	1	0.0%
tapdb/assets_store.go	4	7	57.14%
taprpc/tapchannelrpc/tapchannel.pb.go	0	5	0.0%
tapchannel/aux_funding_controller.go	0	68	0.0%

Files with Coverage Reduction	New Missed Lines	%
asset/group_key.go	2	72.65%
fn/iter.go	2	62.07%
tapchannel/aux_leaf_signer.go	2	43.08%
tapdb/assets_common.go	2	78.39%
tapgarden/custodian.go	2	74.33%
universe_rpc_diff.go	2	76.0%
itest/multisig.go	3	97.43%
tapdb/universe.go	4	81.76%
tapgarden/caretaker.go	4	77.14%
universe/syncer.go	5	85.68%

Totals
Change from base Build 13547766504:	-0.05%
Covered Lines:	48914
Relevant Lines:	89714

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💛 - Coveralls