Create EncryptionProvider trait #455

Job	Run time
build_attest_all (buildconfigs/key_xor_test_app.sh)	3m 46s
build_attest_all (buildconfigs/oak_client_android_app.sh)	50m 11s
build_attest_all (buildconfigs/oak_containers_agent.sh)	5m 30s
build_attest_all (buildconfigs/oak_containers_kernel.sh)	27m 30s
build_attest_all (buildconfigs/oak_containers_orchestrator.sh)	7m 48s
build_attest_all (buildconfigs/oak_containers_stage1.sh)	5m 11s
build_attest_all (buildconfigs/oak_containers_syslogd.sh)	6m 54s
build_attest_all (buildconfigs/oak_containers_system_image.sh)	14m 0s
build_attest_all (buildconfigs/oak_containers_nvidia_system_image.sh)	15m 38s
build_attest_all (buildconfigs/oak_echo_enclave_app.sh)	4m 3s
build_attest_all (buildconfigs/oak_echo_raw_enclave_app.sh)	3m 36s
build_attest_all (buildconfigs/oak_functions_enclave_app.sh)	4m 14s
build_attest_all (buildconfigs/oak_functions_insecure_enclave_app.sh)	3m 45s
build_attest_all (buildconfigs/oak_orchestrator.sh)	3m 57s
build_attest_all (buildconfigs/oak_restricted_kernel_simple_io_init_rd_wrapper_bin.sh)	16m 25s
build_attest_all (buildconfigs/stage0_bin.sh)	39m 45s
	3h 32m 13s

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