add a variant to ingest benchmark with shard-splitting disabled (#10876)

## Problem

we measure ingest performance for a few variants (stripe-sizes,
pre-sharded, shard-splitted).
However some phenomena (e.g. related to L0 compaction) in PS can be
better observed and optimized with un-sharded tenants.

## Summary of changes

- Allow to create projects with a policy that disables sharding
(`{"scheduling": "Essential"}`)
- add a variant to ingest_benchmark that uses that policy for the new
project

## Test run
https://github.com/neondatabase/neon/actions/runs/13396325970

.github/workflows/ingest_benchmark.yml

@@ -32,18 +32,27 @@ jobs:
           - target_project: new_empty_project_stripe_size_2048 
             stripe_size: 2048 # 16 MiB
             postgres_version: 16
+            disable_sharding: false
           - target_project: new_empty_project_stripe_size_32768
             stripe_size: 32768 # 256 MiB # note that this is different from null because using null will shard_split the project only if it reaches the threshold
                                # while here it is sharded from the beginning with a shard size of 256 MiB
+            disable_sharding: false
             postgres_version: 16
           - target_project: new_empty_project
             stripe_size: null # run with neon defaults which will shard split only when reaching the threshold
+            disable_sharding: false
             postgres_version: 16
           - target_project: new_empty_project
             stripe_size: null # run with neon defaults which will shard split only when reaching the threshold
+            disable_sharding: false
             postgres_version: 17
           - target_project: large_existing_project
             stripe_size: null # cannot re-shared or choose different stripe size for existing, already sharded project
+            disable_sharding: false
+            postgres_version: 16
+          - target_project: new_empty_project_unsharded
+            stripe_size: null # run with neon defaults which will shard split only when reaching the threshold
+            disable_sharding: true
             postgres_version: 16
       max-parallel: 1 # we want to run each stripe size sequentially to be able to compare the results
     permissions:
@@ -96,6 +105,7 @@ jobs:
         admin_api_key: ${{ secrets.NEON_STAGING_ADMIN_API_KEY }} 
         shard_count: 8
         stripe_size: ${{ matrix.stripe_size }}
+        disable_sharding: ${{ matrix.disable_sharding }} 
 
     - name: Initialize Neon project
       if: ${{ startsWith(matrix.target_project, 'new_empty_project') }}