Add Oak Session Low-Level SDK stub #21

Job	Run time
build_attest_all (buildconfigs/key_xor_test_app.sh)	2m 13s
build_attest_all (buildconfigs/oak_containers_kernel.sh)	11m 7s
build_attest_all (buildconfigs/oak_containers_orchestrator.sh)	6m 33s
build_attest_all (buildconfigs/oak_containers_stage1.sh)	3m 49s
build_attest_all (buildconfigs/oak_containers_syslogd.sh)	5m 58s
build_attest_all (buildconfigs/oak_containers_system_image.sh)	11m 56s
build_attest_all (buildconfigs/oak_echo_enclave_app.sh)	2m 19s
build_attest_all (buildconfigs/oak_echo_raw_enclave_app.sh)	2m 15s
build_attest_all (buildconfigs/oak_functions_enclave_app.sh)	2m 14s
build_attest_all (buildconfigs/oak_functions_insecure_enclave_app.sh)	2m 29s
build_attest_all (buildconfigs/oak_restricted_kernel_simple_io_init_rd_wrapper_bin.sh)	2m 51s
build_attest_all (buildconfigs/stage0_bin.sh)	2m 20s
build_attest_all (buildconfigs/oak_orchestrator.sh)	2m 6s
	58m 10s

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