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.

libs/pageserver_api/src/models.rs

@@ -435,6 +435,8 @@ pub struct TenantShardLocation {
 #[serde(deny_unknown_fields)]
 pub struct TenantLocationConfigResponse {
     pub shards: Vec<TenantShardLocation>,
+    // If the shards' ShardCount count is >1, stripe_size will be set.
+    pub stripe_size: Option<ShardStripeSize>,
 }
 
 #[derive(Serialize, Deserialize, Debug)]