storage controller: include stripe size in compute notifications (#6974)

## Problem

- The storage controller is the source of truth for a tenant's stripe
size, but doesn't currently have a way to propagate that to compute:
we're just using the default stripe size everywhere.

Closes: https://github.com/neondatabase/neon/issues/6903

## Summary of changes

- Include stripe size in `ComputeHookNotifyRequest`
- Include stripe size in `LocationConfigResponse`

The stripe size is optional: it will only be advertised for
multi-sharded tenants. This enables the controller to defer the choice
of stripe size until we split a tenant for the first time.

pageserver/src/http/openapi_spec.yml

@@ -1339,6 +1339,10 @@ components:
           type: array
           items:
             $ref: "#/components/schemas/TenantShardLocation"
+        stripe_size:
+          description: If multiple shards are present, this field contains the sharding stripe size, else it is null.
+          type: integer
+          nullable: true
     TenantShardLocation:
       type: object
       required:

pageserver/src/http/routes.rs

@@ -1451,11 +1451,12 @@ async fn put_tenant_location_config_handler(
         tenant::SpawnMode::Eager
     };
 
-    let attached = state
+    let tenant = state
         .tenant_manager
         .upsert_location(tenant_shard_id, location_conf, flush, spawn_mode, &ctx)
-        .await?
-        .is_some();
+        .await?;
+    let stripe_size = tenant.as_ref().map(|t| t.get_shard_stripe_size());
+    let attached = tenant.is_some();
 
     if let Some(_flush_ms) = flush {
         match state
@@ -1477,12 +1478,20 @@ async fn put_tenant_location_config_handler(
     // This API returns a vector of pageservers where the tenant is attached: this is
     // primarily for use in the sharding service.  For compatibilty, we also return this
     // when called directly on a pageserver, but the payload is always zero or one shards.
-    let mut response = TenantLocationConfigResponse { shards: Vec::new() };
+    let mut response = TenantLocationConfigResponse {
+        shards: Vec::new(),
+        stripe_size: None,
+    };
     if attached {
         response.shards.push(TenantShardLocation {
             shard_id: tenant_shard_id,
             node_id: state.conf.id,
-        })
+        });
+        if tenant_shard_id.shard_count.count() > 1 {
+            // Stripe size should be set if we are attached
+            debug_assert!(stripe_size.is_some());
+            response.stripe_size = stripe_size;
+        }
     }
 
     json_response(StatusCode::OK, response)