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/TEST_MAPPING b/TEST_MAPPING
index 7be81a8..25908ad 100644
--- a/TEST_MAPPING
+++ b/TEST_MAPPING
@@ -17,6 +17,9 @@
     },
     {
       "name": "compsvc_device_tests"
+    },
+    {
+      "name": "compos_key_tests"
     }
   ],
   "postsubmit": [