[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 13571552700

Details

• 14 of 93 (15.05%) changed or added relevant lines in 7 files are covered.
• No unchanged relevant lines lost coverage.
• Overall first build on group-key-support at 54.566%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
rpcserver.go	0	1	0.0%
tapchannelmsg/custom_channel_data.go	0	2	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%

Totals
Change from base Build 13571552842:	54.6%
Covered Lines:	49008
Relevant Lines:	89814

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💛 - Coveralls