btcutil: Add library functions for BIP-0352 silent payment _send_ support

[guggero]

Adds library/API functionality for Silent Payment send support.

Implements the following BIPs (or pull requests to BIPs):

• BIP-0352 - Silent Payments (added in BIP 352: Silent Payments bitcoin/bips#1458)
• BIP-0374 - Discrete Log Equality Proofs (added in BIP374: Discrete Log Equality Proofs (DLEQ) bitcoin/bips#1689)
• BIP-0375 - Sending Silent Payments with PSBTs (added in BIP375: Sending Silent Payments in PSBTs bitcoin/bips#1687)

Goal is to address send support first as that's rather straightforward. Receive support is much more involved as it requires scanning the chain in a specific manner.

[coveralls]

Pull Request Test Coverage Report for Build 14137608998

Details

• 515 of 722 (71.33%) changed or added relevant lines in 9 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.3%) to 56.803%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
btcutil/silentpayments/output.go	52	58	89.66%
btcutil/silentpayments/secp.go	40	51	78.43%
btcutil/psbt/partial_output.go	27	39	69.23%
btcutil/psbt/partial_input.go	32	46	69.57%
btcutil/psbt/psbt.go	40	54	74.07%
btcutil/psbt/silentpayments.go	40	58	68.97%
btcutil/silentpayments/input.go	81	102	79.41%
btcutil/silentpayments/dleq.go	104	131	79.39%
btcutil/silentpayments/address.go	99	183	54.1%

Files with Coverage Reduction	New Missed Lines	%
peer/peer.go	1	74.38%

Totals
Change from base Build 14076739198:	0.3%
Covered Lines:	31572
Relevant Lines:	55582

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💛 - Coveralls