Add a test seeder behind the unsafe_test_mode feature

For the end-to-end tests: with the feature, /test/seeder answers the calls the
SDK's live tests make to a seeder API (POST /seed with SingleUserScene or
TestVectorUserScene, DELETE /seed/<playId>), behind HTTP basic auth from
TEST_SEEDER_USERNAME and TEST_SEEDER_PASSWORD, and 404 without them.

It creates accounts without any checks, so no release build has the feature.
CI builds and tests it with sqlite.

src/api/core/mod.rs

@@ -14,6 +14,8 @@ pub use ciphers::{CipherData, CipherSyncData, CipherSyncType, purge_trashed_ciph
 pub use emergency_access::{emergency_notification_reminder_job, emergency_request_timeout_job};
 pub use events::{event_cleanup_job, log_event, log_user_event};
 pub use sends::purge_sends;
+#[cfg(feature = "unsafe_test_mode")]
+pub use {ciphers::seed_cipher, sends::seed_send};
 
 use reqwest::Method;
 use rocket::{Catcher, Route, serde::json::Json, serde::json::Value};