collab: Add usages table to LLM database (#15884)

This PR adds a `usages` table to the LLM database.

We'll use this to track usage for rate-limiting purposes.

Release Notes:

- N/A

crates/collab/src/llm/db/tests/usage_tests.rs

@@ -0,0 +1,24 @@
+use std::sync::Arc;
+
+use pretty_assertions::assert_eq;
+use rpc::LanguageModelProvider;
+
+use crate::llm::db::LlmDatabase;
+use crate::test_both_llm_dbs;
+
+test_both_llm_dbs!(
+    test_find_or_create_usage,
+    test_find_or_create_usage_postgres,
+    test_find_or_create_usage_sqlite
+);
+
+async fn test_find_or_create_usage(db: &Arc<LlmDatabase>) {
+    db.initialize_providers().await.unwrap();
+
+    let usage = db
+        .find_or_create_usage(123, LanguageModelProvider::Anthropic, "claude-3-5-sonnet")
+        .await
+        .unwrap();
+
+    assert_eq!(usage.user_id, 123);
+}