proof: add signed universe commitment params

[guggero]

Depends on #1335, only last commit is new.

Adds a new struct that contains the universe commitment parameters and a signature over it.

TODOs:

• Allow commitment params to have unknown odd records
• Add the struct to the main Proof type

[coveralls]

Pull Request Test Coverage Report for Build 13412695300

Details

• 151 of 206 (73.3%) changed or added relevant lines in 2 files are covered.
• 34 unchanged lines in 7 files lost coverage.
• Overall coverage increased (+0.07%) to 54.527%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
proof/encoding.go	32	44	72.73%
proof/uni_commitments.go	119	162	73.46%

Files with Coverage Reduction	New Missed Lines	%
commitment/tap.go	1	85.0%
asset/group_key.go	2	72.65%
tapgarden/custodian.go	2	75.27%
rfqmsg/records.go	3	71.2%
universe/base.go	3	79.25%
tapchannel/aux_leaf_signer.go	5	43.08%
address/mock.go	18	86.93%

Totals
Change from base Build 13397714787:	0.07%
Covered Lines:	48794
Relevant Lines:	89486

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💛 - Coveralls