descriptors: add a method to get a PSBT's change outputs #1678

Job	Run time
linter	34s
linter_gui	3m 46s
unit_tests (1.63, ubuntu-latest)	3m 5s
unit_tests (1.63, macOS-latest)	5m 14s
unit_tests (1.63, windows-latest)	4m 23s
unit_tests (nightly, ubuntu-latest)	53s
unit_tests (nightly, macOS-latest)	1m 47s
unit_tests (nightly, windows-latest)	1m 46s
unit_tests_gui (1.65, ubuntu-latest)	4m 58s
unit_tests_gui (1.65, macOS-latest)	10m 43s
unit_tests_gui (1.65, windows-latest)	6m 31s
unit_tests_gui (nightly, ubuntu-latest)	2m 31s
unit_tests_gui (nightly, macOS-latest)	2m 58s
unit_tests_gui (nightly, windows-latest)	4m 37s
	53m 46s

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