Declare that oauth client credentials are using POST method

Not enforced by mod_http_oauth2, but could be in the future

snikket_web/prosodyclient.py

@@ -474,6 +474,7 @@ class ProsodyClient:
             ],
             "grant_types": ["password"],
             "response_types": [],
+            "token_endpoint_auth_method": "client_secret_post",
             "scope": " ".join([SCOPE_RESTRICTED, SCOPE_DEFAULT, SCOPE_ADMIN]),
             "software_version": _version.version,
         }