pageserver: avoid reading SLRU blocks for GC on shards >0 (#9423)

## Problem

SLRU blocks, which can add up to several gigabytes, are currently
ingested by all shards, multiplying their capacity cost by the shard
count and slowing down ingest. We do this because all shards need the
SLRU pages to do timestamp->LSN lookup for GC.

Related: https://github.com/neondatabase/neon/issues/7512

## Summary of changes

- On non-zero shards, learn the GC offset from shard 0's index instead
of calculating it.
- Add a test `test_sharding_gc` that exercises this
- Do GC in test_pg_regress as a general smoke test that GC functions run
(e.g. this would fail if we were using SLRUs we didn't have)

In this PR we are still ingesting SLRUs everywhere, but not using them
any more. Part 2 PR (https://github.com/neondatabase/neon/pull/9786)
makes the change to not store them at all.

## Checklist before requesting a review

- [ ] I have performed a self-review of my code.
- [ ] If it is a core feature, I have added thorough tests.
- [ ] Do we need to implement analytics? if so did you add the relevant
metrics to the dashboard?
- [ ] If this PR requires public announcement, mark it with
/release-notes label and add several sentences in this section.

## Checklist before merging

- [ ] Do not forget to reformat commit message to not include the above
checklist

test_runner/fixtures/remote_storage.py

@@ -77,14 +77,16 @@ class MockS3Server:
 class LocalFsStorage:
     root: Path
 
-    def tenant_path(self, tenant_id: TenantId) -> Path:
+    def tenant_path(self, tenant_id: Union[TenantId, TenantShardId]) -> Path:
         return self.root / "tenants" / str(tenant_id)
 
-    def timeline_path(self, tenant_id: TenantId, timeline_id: TimelineId) -> Path:
+    def timeline_path(
+        self, tenant_id: Union[TenantId, TenantShardId], timeline_id: TimelineId
+    ) -> Path:
         return self.tenant_path(tenant_id) / "timelines" / str(timeline_id)
 
     def timeline_latest_generation(
-        self, tenant_id: TenantId, timeline_id: TimelineId
+        self, tenant_id: Union[TenantId, TenantShardId], timeline_id: TimelineId
     ) -> Optional[int]:
         timeline_files = os.listdir(self.timeline_path(tenant_id, timeline_id))
         index_parts = [f for f in timeline_files if f.startswith("index_part")]
@@ -102,7 +104,9 @@ class LocalFsStorage:
             raise RuntimeError(f"No index_part found for {tenant_id}/{timeline_id}")
         return generations[-1]
 
-    def index_path(self, tenant_id: TenantId, timeline_id: TimelineId) -> Path:
+    def index_path(
+        self, tenant_id: Union[TenantId, TenantShardId], timeline_id: TimelineId
+    ) -> Path:
         latest_gen = self.timeline_latest_generation(tenant_id, timeline_id)
         if latest_gen is None:
             filename = TIMELINE_INDEX_PART_FILE_NAME
@@ -126,7 +130,9 @@ class LocalFsStorage:
         filename = f"{local_name}-{generation:08x}"
         return self.timeline_path(tenant_id, timeline_id) / filename
 
-    def index_content(self, tenant_id: TenantId, timeline_id: TimelineId) -> Any:
+    def index_content(
+        self, tenant_id: Union[TenantId, TenantShardId], timeline_id: TimelineId
+    ) -> Any:
         with self.index_path(tenant_id, timeline_id).open("r") as f:
             return json.load(f)