feat(schedule)!: move schedule parser, downloader, and updater to external library

This can be used to support more schedule formats in the future.

providers/src/lib.rs

@@ -0,0 +1,9 @@
+pub use base;
+
+pub use provider_engels_polytechnic::EngelsPolytechnicProvider;
+pub use provider_engels_polytechnic::UpdateSource as EngelsPolytechnicUpdateSource;
+
+#[cfg(feature = "test")]
+pub mod test_utils {
+    pub use provider_engels_polytechnic::test_utils as engels_polytechnic;
+}