providers/oauth2: optimise and cache signing key, prevent key being loaded multiple times

Signed-off-by: Jens Langhammer <jens.langhammer@beryju.org>

authentik/providers/oauth2/tests/test_token_cc_jwt_source.py

@@ -210,7 +210,7 @@ class TestTokenClientCredentialsJWTSource(OAuthTestCase):
         self.assertEqual(response.status_code, 200)
         body = loads(response.content.decode())
         self.assertEqual(body["token_type"], "bearer")
-        _, alg = self.provider.get_jwt_key()
+        _, alg = self.provider.jwt_key
         jwt = decode(
             body["access_token"],
             key=self.provider.signing_key.public_key,