rpc: add SetPendingTransferProofCourierAddr endpoint to fix erroneous courier addresses

[ffranr]

This PR introduces a new RPC endpoint, SetPendingTransferProofCourierAddr, which allows overriding the proof courier address for a pending asset transfer output. This enhancement directly addresses the issue of correcting erroneous courier addresses during asset transfers as described in issue #1082.

[ffranr]

I'm looking for early feedback on the general direction of this PR.

This PR is in draft for the following reasons:

• Missing itest.
• Updated proof courier address is not updated on dist (stored to the database).

[coveralls]

Pull Request Test Coverage Report for Build 10490103886

Details

• 0 of 76 (0.0%) changed or added relevant lines in 2 files are covered.
• 20 unchanged lines in 6 files lost coverage.
• Overall coverage decreased (-0.06%) to 40.353%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tapfreighter/chain_porter.go	0	5	0.0%
rpcserver.go	0	71	0.0%
<!--	Total:	0	76

Files with Coverage Reduction	New Missed Lines	%
rpcserver.go	1	0.0%
tappsbt/create.go	2	53.22%
asset/asset.go	2	81.35%
tapdb/universe.go	4	80.91%
tapgarden/caretaker.go	4	68.5%
tapdb/multiverse.go	7	60.32%
<!--	Total:	20

Totals
Change from base Build 10477817019:	-0.06%
Covered Lines:	23942
Relevant Lines:	59331

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💛 - Coveralls