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/Android.bp b/compos/Android.bp
index b9fcfff..783ba22 100644
--- a/compos/Android.bp
+++ b/compos/Android.bp
@@ -50,6 +50,8 @@
         "liblog_rust",
         "libminijail_rust",
         "libnix",
+        "libodsign_proto_rust",
+        "libprotobuf",
         "libring",
         "libscopeguard",
     ],