Add code to bulk-sign artifacts

This isn't wired up yet - we need to be able to iterate over AuthFs
directories, but that's coming soon.

Import the OdsignInfo proto, which contains the map of filenames and
digests, and provide a way to build it from a set of artifact files
and write it along with its signature.

Also remove the old Signer trait (which is unused), and a couple of
small refactorings.

Bug: 161471326
Test: Builds
Change-Id: I37fccb1f2ca4fa1ea3006185d9f805d668252e2a
diff --git a/compos/src/compsvc_main.rs b/compos/src/compsvc_main.rs
index fc00039..9347905 100644
--- a/compos/src/compsvc_main.rs
+++ b/compos/src/compsvc_main.rs
@@ -16,11 +16,11 @@
 
 //! A tool to start a standalone compsvc server that serves over RPC binder.
 
+mod artifact_signer;
 mod compilation;
 mod compos_key_service;
 mod compsvc;
 mod fsverity;
-mod signer;
 
 use android_system_virtualmachineservice::{
     aidl::android::system::virtualmachineservice::IVirtualMachineService::{