Add centralized C++ utilities for oak::Variant. #719

Job	Run time
build_attest_all (buildconfigs/key_xor_test_app.sh)	18m 37s
build_attest_all (buildconfigs/oak_client_android_app.sh)	51m 35s
build_attest_all (buildconfigs/oak_containers_agent.sh)	12m 44s
build_attest_all (buildconfigs/oak_containers_kernel.sh)	32m 38s
build_attest_all (buildconfigs/oak_containers_orchestrator.sh)	24m 42s
build_attest_all (buildconfigs/oak_containers_stage1.sh)	24m 32s
build_attest_all (buildconfigs/oak_containers_syslogd.sh)	23m 56s
build_attest_all (buildconfigs/oak_containers_system_image.sh)	26m 0s
build_attest_all (buildconfigs/oak_containers_nvidia_system_image.sh)	26m 16s
build_attest_all (buildconfigs/oak_echo_enclave_app.sh)	18m 42s
build_attest_all (buildconfigs/oak_echo_raw_enclave_app.sh)	18m 17s
build_attest_all (buildconfigs/oak_functions_enclave_app.sh)	18m 42s
build_attest_all (buildconfigs/oak_functions_insecure_enclave_app.sh)	18m 36s
build_attest_all (buildconfigs/oak_orchestrator.sh)	18m 41s
build_attest_all (buildconfigs/oak_restricted_kernel_simple_io_init_rd_wrapper_bin.sh)	42m 7s
build_attest_all (buildconfigs/stage0_bin.sh)	43m 19s
build_attest_all (buildconfigs/stage0_bin_tdx.sh)	41m 53s
	7h 41m 17s

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