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/retained_wal.sql

@@ -0,0 +1,5 @@
+SELECT
+  slot_name,
+  pg_wal_lsn_diff(pg_current_wal_lsn(), restart_lsn)::FLOAT8 AS retained_wal
+FROM pg_replication_slots
+WHERE active = false;