Key blob protection using AEAD

Add BlobEncryptor, which can be used to write a secret (like our
private key) to an encrypted & authenticated blob and later retrieve
it.

Added the skeleton of code to make use of this, using the sealing CDI
as the input to the key derivation.

Bug: 214233409
Test: atest compsvc_device_tests
Change-Id: Iea7e82405072a31ae5f7ad64a9e894a970913219
diff --git a/compos/src/compsvc_main.rs b/compos/src/compsvc_main.rs
index b4e3128..23a6ed0 100644
--- a/compos/src/compsvc_main.rs
+++ b/compos/src/compsvc_main.rs
@@ -17,11 +17,13 @@
 //! A tool to start a standalone compsvc server that serves over RPC binder.
 
 mod artifact_signer;
+mod blob_encryptor;
 mod compilation;
 mod compos_key_service;
 mod compsvc;
 mod dice;
 mod fsverity;
+mod signing_key;
 
 use android_system_virtualmachineservice::{
     aidl::android::system::virtualmachineservice::IVirtualMachineService::{