Sign artifacts after compilation

If compilation succeeds, pass over all the artifacts we generated and
produce an OdsignInfo file with all the names and digests and a
signature of it using the CompOS key.

Various refactorings along the way to make it eaiser:
- Extract the odrefresh ExitCode enum for reuse
- Extracted a Signer, distinct from an ArtifactSigner
- Defined OdrefreshContext to stop the linter complaining about a
  function with >7 arguments.

Also added more debugging to help fix my various silly mistakes.

Bug: 161471326
Test: composd_cmd forced-odrefresh -> signature file produced
Change-Id: Ia6a6abd9e28158f4767e7752727635087f9bb566
diff --git a/compos/common/lib.rs b/compos/common/lib.rs
index 9b07030..5c28379 100644
--- a/compos/common/lib.rs
+++ b/compos/common/lib.rs
@@ -17,6 +17,7 @@
 //! Common items used by CompOS server and/or clients
 
 pub mod compos_client;
+pub mod odrefresh;
 pub mod timeouts;
 
 /// Special CID indicating "any".