refactor: make `Descriptor` plan API available for generic `Pk` #2575

Job	Run time
Docs - stable toolchain	37s
Docs - nightly toolchain	49s
Test - nightly toolchain (recent)	10m 21s
Test - stable toolchain (minimal)	11m 36s
Lint - nightly toolchain (recent)	1m 8s
Test - 1.63.0 toolchain (recent)	23m 15s
Test - 1.63.0 toolchain (minimal)	22m 19s
Test - nightly toolchain (minimal)	12m 0s
Test - stable toolchain (recent)	12m 8s
Bench - nightly toolchain	4m 10s
Format - nightly toolchain	45s
Embedded	1m 32s
Integration tests - stable toolchain (28_2)	3m 6s
Integration tests - stable toolchain (29_0)	2m 53s
Integration tests - stable toolchain (0_21_2)	3m 12s
Integration tests - stable toolchain (27_2)	2m 54s
Integration tests - stable toolchain (24_2)	3m 5s
Integration tests - stable toolchain (23_2)	3m 9s
Integration tests - stable toolchain (0_19_1)	3m 17s
Integration tests - stable toolchain (26_2)	3m 18s
Integration tests - stable toolchain (0_20_2)	2m 59s
Integration tests - stable toolchain (25_2)	4m 32s
Integration tests - stable toolchain (0_18_1)	9m 9s
Integration tests - stable toolchain (22_1)	3m 20s
Integration tests - stable toolchain (0_17_2)	2m 54s
	2h 28m 28s

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