Cache web authentication results

Add a short-lived, HMAC-keyed cache for validated principals and
invalidate entries when principals become stale. Improve duplicate SSH
key
errors and enable Ed25519 streaming verification.

src/user_database.c

@@ -1175,7 +1175,7 @@ esp_err_t user_database_add_ssh_key(
                     key->blob_length == key_blob_length &&
                     memcmp(key->type, key_type, key_type_length) == 0 &&
                     constant_time_equal(key->blob, key_blob, key_blob_length)) {
-                    error = ESP_ERR_INVALID_STATE;
+                    error = USER_DATABASE_ERR_DUPLICATE_SSH_KEY;
                     break;
                 }
             }