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/checkpoints_timed.libsonnet

@@ -0,0 +1,10 @@
+{
+  metric_name: 'checkpoints_timed',
+  type: 'gauge',
+  help: 'Number of scheduled checkpoints',
+  key_labels: null,
+  values: [
+    'checkpoints_timed',
+  ],
+  query: importstr 'sql_exporter/checkpoints_timed.sql',
+}