feat(spooler): Add EnvelopeStack based on SQLite

migrations/20240724144200_change_index.sql

@@ -0,0 +1,3 @@
+DROP INDEX IF EXISTS project_keys;
+
+CREATE INDEX IF NOT EXISTS project_keys_received_at ON envelopes (own_key, sampling_key, received_at);

relay-server/src/services/spooler/envelope_stack/mod.rs

@@ -0,0 +1,16 @@
+use crate::envelope::Envelope;
+
+mod sqlite;
+
+pub trait EnvelopeStack {
+    type Error;
+
+    #[allow(dead_code)]
+    async fn push(&mut self, envelope: Box<Envelope>) -> Result<(), Self::Error>;
+
+    #[allow(dead_code)]
+    async fn peek(&mut self) -> Result<&Box<Envelope>, Self::Error>;
+
+    #[allow(dead_code)]
+    async fn pop(&mut self) -> Result<Box<Envelope>, Self::Error>;
+}