compos: Sign fs-verity digest of output files

compsvc now needs to be initialized with the signing key blob to finish
the whole compilation flow, otherwise it'll be an illegal state error.
"init-key" command is added to compos_key_cmd for that purpose.

Also, move some fs-verity related code into fsverity.rs.

Bug: 161471326
Test: ComposHostTestCases
Change-Id: I80db78ce11dc1f49e9ee36af47ad98f200aa4388
diff --git a/compos/src/compsvc_main.rs b/compos/src/compsvc_main.rs
index 48e37b6..6396556 100644
--- a/compos/src/compsvc_main.rs
+++ b/compos/src/compsvc_main.rs
@@ -21,6 +21,7 @@
 mod compilation;
 mod compos_key_service;
 mod compsvc;
+mod fsverity;
 mod signer;
 
 use crate::common::{SERVICE_NAME, VSOCK_PORT};