refactor: structure of an hexagonal architecture

Created a kernel crate to store models and future action implementations.
Will be useful to create admin cli.

tests/manual/access_token_request.sh

@@ -0,0 +1,7 @@
+#!/usr/bin/sh
+
+curl -v http://localhost:8085/api/token \
+    -u "a1785786-8be1-443c-9a6f-35feed703609":"49c6c16a-0a8a-4981-a60d-5cb96582cc1a" \
+    -d grant_type="authorization_code" \
+    -d code="$(cat tmp/authorize_code.txt)" \
+    -d redirect_uri="http://localhost:9090/authorize" > tmp/access_token.json