tapd: allow skipping the funding step in CommitVirtualPsbts

[bhandras]

In order to be able to construct zero fee transactions (which do not need funding inputs) we want to allow CommitVirtualPsbts to skip the funding step altogether.

[coveralls]

Pull Request Test Coverage Report for Build 15272152109

Details

• 61 of 76 (80.26%) changed or added relevant lines in 2 files are covered.
• 35 unchanged lines in 6 files lost coverage.
• Overall coverage increased (+18.6%) to 55.795%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
taprpc/assetwalletrpc/assetwallet.pb.go	0	5	0.0%
rpcserver.go	61	71	85.92%

Files with Coverage Reduction	New Missed Lines	%
asset/asset.go	2	79.49%
tapchannel/aux_leaf_signer.go	2	43.08%
commitment/tap.go	3	84.55%
asset/mock.go	6	71.16%
tapdb/multiverse.go	6	82.23%
address/mock.go	16	88.24%

Totals
Change from base Build 15255103485:	18.6%
Covered Lines:	52736
Relevant Lines:	94518

---

💛 - Coveralls

[guggero]

LGTM, just need to address the linter complaints.

[GeorgeTsagk]

LGTM, would like some test coverage but since it's involved I think it's OK to merge as is, given that it's a noop if you ignore the flag