storage scrubber: GC ancestor shard layers (#8196)

## Problem

After a shard split, the pageserver leaves the ancestor shard's content
in place. It may be referenced by child shards, but eventually child
shards will de-reference most ancestor layers as they write their own
data and do GC. We would like to eventually clean up those ancestor
layers to reclaim space.

## Summary of changes

- Extend the physical GC command with `--mode=full`, which includes
cleaning up unreferenced ancestor shard layers
- Add test `test_scrubber_physical_gc_ancestors`
- Remove colored log output: in testing this is irritating ANSI code
spam in logs, and in interactive use doesn't add much.
- Refactor storage controller API client code out of storcon_client into
a `storage_controller/client` crate
- During physical GC of ancestors, call into the storage controller to
check that the latest shards seen in S3 reflect the latest state of the
tenant, and there is no shard split in progress.

libs/utils/src/auth.rs

@@ -33,6 +33,10 @@ pub enum Scope {
     GenerationsApi,
     // Allows access to control plane managment API and some storage controller endpoints.
     Admin,
+
+    /// Allows access to storage controller APIs used by the scrubber, to interrogate the state
+    /// of a tenant & post scrub results.
+    Scrubber,
 }
 
 /// JWT payload. See docs/authentication.md for the format