Generate sql_exporter config files with Jsonnet

There are quite a few benefits to this approach:

- Reduce config duplication
  - The two sql_exporter configs were super similar with just a few
    differences
- Pull SQL queries into standalone files
  - That means we could run a SQL formatter on the file in the future
  - It also means access to syntax highlighting
- In the future, run different queries for different PG versions
  - This is relevant because right now, we have queries that are failing
    on PG 17 due to catalog updates

Signed-off-by: Tristan Partin <tristan@neon.tech>

compute/etc/sql_exporter/replication_delay_seconds.sql

@@ -0,0 +1,5 @@
+SELECT
+  CASE
+    WHEN pg_last_wal_receive_lsn() = pg_last_wal_replay_lsn() THEN 0
+    ELSE GREATEST(0, EXTRACT (EPOCH FROM now() - pg_last_xact_replay_timestamp()))
+  END AS replication_delay_seconds;