feat: add GetThreads API #5462

Job	Run time
cross-i386	1m 20s
cross-i386 (runc_nodmz)	1m 35s
test (ubuntu-20.04, 1.20.x, rootless, -race)	6m 1s
test (ubuntu-20.04, 1.20.x, rootless)	5m 25s
test (ubuntu-20.04, 1.20.x, -race)	6m 26s
test (ubuntu-20.04, 1.20.x)	5m 28s
test (ubuntu-20.04, 1.21.x, rootless, -race)	5m 54s
test (ubuntu-20.04, 1.21.x, rootless)	5m 21s
test (ubuntu-20.04, 1.21.x, -race)	6m 14s
test (ubuntu-20.04, 1.21.x)	5m 26s
test (ubuntu-20.04, 1.21.x, criu-dev)	6m 16s
test (ubuntu-22.04, 1.20.x, rootless, -race)	9m 36s
test (ubuntu-22.04, 1.20.x, rootless, -race, runc_nodmz)	9m 31s
test (ubuntu-22.04, 1.20.x, rootless)	9m 25s
test (ubuntu-22.04, 1.20.x, rootless, runc_nodmz)	9m 6s
test (ubuntu-22.04, 1.20.x, -race)	7m 20s
test (ubuntu-22.04, 1.20.x, -race, runc_nodmz)	7m 28s
test (ubuntu-22.04, 1.20.x)	6m 53s
test (ubuntu-22.04, 1.20.x, runc_nodmz)	6m 36s
test (ubuntu-22.04, 1.21.x, rootless, -race)	9m 42s
test (ubuntu-22.04, 1.21.x, rootless, -race, runc_nodmz)	9m 27s
test (ubuntu-22.04, 1.21.x, rootless)	9m 25s
test (ubuntu-22.04, 1.21.x, rootless, runc_nodmz)	9m 24s
test (ubuntu-22.04, 1.21.x, -race)	7m 25s
test (ubuntu-22.04, 1.21.x, -race, runc_nodmz)	7m 22s
test (ubuntu-22.04, 1.21.x)	6m 33s
test (ubuntu-22.04, 1.21.x, runc_nodmz)	7m 1s
test (ubuntu-22.04, 1.21.x, criu-dev)	8m 29s
	3h 16m 9s

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