Add support for bearer/token auth

New options added to configuration file to reference a public key used
to validate authorization tokens signed by an auth server with
corresponding private key.

Resolves #24

Signed-off-by: Peter Engelbert <pmengelbert@gmail.com>

examples/config-bearer-auth.json

@@ -0,0 +1,20 @@
+{
+  "version":"0.1.0-dev",
+  "storage":{
+    "rootDirectory":"/tmp/zot"
+  },
+  "http": {
+    "address":"127.0.0.1",
+    "port":"8080",
+    "auth": {
+      "bearer": {
+        "realm": "https://auth.myreg.io/auth/token",
+        "service": "myauth",
+        "cert": "/etc/zot/auth.crt"
+      }
+    }
+  },
+  "log":{
+    "level":"debug"
+  }
+}