psbt: add verification method for utxo data

[ziggie1984]

This adds a method to verify the utxo data of a psbt package especially the NonWitnessUtxo | WitnessUtxo data.

This will be used in other packages to make sure we have the correct psbt structure with all relevant input fields populated.

Relates to lightningnetwork/lnd#7527

Happy to discuss other approaches, other PRs which call this function in lnd and btcwallet would follow if we want this function in this package ?

[ziggie1984]

Thank you for taking a look so fast, really appreciate this :)

[coveralls]

Pull Request Test Coverage Report for Build 4476808113

• 0 of 12 (0.0%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.01%) to 55.272%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
btcutil/psbt/utils.go	0	12	0.0%
<!--	Total:	0	12	0.0%	-->

Files with Coverage Reduction	New Missed Lines	%
mempool/mempool.go	1	66.75%
<!--	Total:	1		-->

Totals
Change from base Build 4326890213:	-0.01%
Covered Lines:	26650
Relevant Lines:	48216

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💛 - Coveralls