backend-{app,plugin}-api: add initial AuthService interface + implementation

Co-authored-by: Fredrik Adelöw <freben@gmail.com>
Co-authored-by: Carl-Erik Bergström <cbergstrom@spotify.com>
Co-authored-by: blam <ben@blam.sh>
Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

packages/backend-app-api/package.json

@@ -65,6 +65,7 @@
     "express-promise-router": "^4.1.0",
     "fs-extra": "^11.2.0",
     "helmet": "^6.0.0",
+    "jose": "^4.6.0",
     "lodash": "^4.17.21",
     "logform": "^2.3.2",
     "minimatch": "^5.0.0",