Add compos_key_helper

Create a small library to do key-related operations (derive, sign,
verify). Add tests.

Create a small standlone executable to expose these functions.

Bug: 218494522
Test: atest compos_key_tests
Change-Id: I5c984178b822510fd32784d01cf4322e592e5d2a
diff --git a/compos/apex/Android.bp b/compos/apex/Android.bp
index aec3c88..6550b4f 100644
--- a/compos/apex/Android.bp
+++ b/compos/apex/Android.bp
@@ -47,6 +47,7 @@
         "composd_cmd",
 
         // Used in VM
+        "compos_key_helper",
         "compsvc",
     ],