Get CompOS talking to diced

Create a module in compsvc to handle using DICE for signing. Initially
we just expose a method for returning our key's attestation
chain.

Add a method to composd, accessed via compos_cmd, to exercise this
functionality for testing purposes.

Bug: 214233409
Test: composd_cmd dice
Change-Id: I65ef19d0126862b800b6539ae1798b1a433085b8
diff --git a/compos/src/compsvc_main.rs b/compos/src/compsvc_main.rs
index 9347905..b4e3128 100644
--- a/compos/src/compsvc_main.rs
+++ b/compos/src/compsvc_main.rs
@@ -20,6 +20,7 @@
 mod compilation;
 mod compos_key_service;
 mod compsvc;
+mod dice;
 mod fsverity;
 
 use android_system_virtualmachineservice::{