Add Signing Key Handle to Oak Containers C++ SDK #74

Job	Run time
build_attest_all (buildconfigs/key_xor_test_app.sh)	2m 20s
build_attest_all (buildconfigs/oak_containers_kernel.sh)	24m 24s
build_attest_all (buildconfigs/oak_containers_orchestrator.sh)	5m 30s
build_attest_all (buildconfigs/oak_containers_stage1.sh)	3m 54s
build_attest_all (buildconfigs/oak_containers_syslogd.sh)	4m 52s
build_attest_all (buildconfigs/oak_containers_system_image.sh)	10m 37s
build_attest_all (buildconfigs/oak_echo_enclave_app.sh)	2m 25s
build_attest_all (buildconfigs/oak_echo_raw_enclave_app.sh)	2m 11s
build_attest_all (buildconfigs/oak_functions_enclave_app.sh)	2m 29s
build_attest_all (buildconfigs/oak_functions_insecure_enclave_app.sh)	2m 25s
build_attest_all (buildconfigs/oak_orchestrator.sh)	2m 19s
build_attest_all (buildconfigs/oak_restricted_kernel_simple_io_init_rd_wrapper_bin.sh)	2m 37s
build_attest_all (buildconfigs/stage0_bin.sh)	2m 56s
	1h 8m 59s

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