Add the ability to configure GenericRemoteStorage for the scrubber (#9652)

Earlier work (#7547) has made the scrubber internally generic, but one
could only configure it to use S3 storage.

This is the final piece to make (most of, snapshotting still requires
S3) the scrubber be able to be configured via GenericRemoteStorage.

I.e. you can now set an env var like:

```
REMOTE_STORAGE_CONFIG='remote_storage = { bucket_name = "neon-dev-safekeeper-us-east-2d", bucket_region = "us-east-2" }
```

and the scrubber will read it instead.

pageserver/ctl/Cargo.toml

@@ -18,7 +18,6 @@ postgres_ffi.workspace = true
 thiserror.workspace = true
 tokio.workspace = true
 tokio-util.workspace = true
-toml_edit.workspace = true
 utils.workspace = true
 svg_fmt.workspace = true
 workspace_hack.workspace = true

pageserver/ctl/src/main.rs

@@ -174,11 +174,7 @@ async fn main() -> anyhow::Result<()> {
                 println!("specified prefix '{}' failed validation", cmd.prefix);
                 return Ok(());
             };
-            let toml_document = toml_edit::DocumentMut::from_str(&cmd.config_toml_str)?;
-            let toml_item = toml_document
-                .get("remote_storage")
-                .expect("need remote_storage");
-            let config = RemoteStorageConfig::from_toml(toml_item)?;
+            let config = RemoteStorageConfig::from_toml_str(&cmd.config_toml_str)?;
             let storage = remote_storage::GenericRemoteStorage::from_config(&config).await;
             let cancel = CancellationToken::new();
             storage