storage: add APIs for warming up location after cold migrations (#10788)

## Problem

We lack an API for warming up attached locations based on the heatmap
contents.
This is problematic in two places:
1. If we manually migrate and cut over while the secondary is still cold
2. When we re-attach a previously offloaded tenant

## Summary of changes

https://github.com/neondatabase/neon/pull/10597 made heatmap generation
additive
across migrations, so we won't clobber it a after a cold migration. This
allows us to implement:

1. An endpoint for downloading all missing heatmap layers on the
pageserver:

`/v1/tenant/:tenant_shard_id/timeline/:timeline_id/download_heatmap_layers`.
Only one such operation per timeline is allowed at any given time. The
granularity is tenant shard.
2. An endpoint to the storage controller to trigger the downloads on the
pageserver:

`/v1/tenant/:tenant_shard_id/timeline/:timeline_id/download_heatmap_layers`.
This works both at
tenant and tenant shard level. If an unsharded tenant id is provided,
the operation is started on
all shards, otherwise only the specified shard.
3. A storcon cli command. Again, tenant and tenant-shard level
granularities are supported.

Cplane will call into storcon and trigger the downloads for all shards.
When we want to rescue a migration, we will use storcon cli targeting
the specific tenant shard.

Related:  https://github.com/neondatabase/neon/issues/10541

libs/utils/src/shard.rs

@@ -117,6 +117,10 @@ impl TenantShardId {
         )
     }
 
+    pub fn range(&self) -> RangeInclusive<Self> {
+        RangeInclusive::new(*self, *self)
+    }
+
     pub fn shard_slug(&self) -> impl std::fmt::Display + '_ {
         ShardSlug(self)
     }