Use DirectClient in BB/BE reconciler

[timebertt]

How to categorize this PR?

/area quality
/kind bug
/priority normal

What this PR does / why we need it:
If the CachedRuntimeClients feature gate is enabled, the backup entry/bucket reconciler in the gardenlet might not execute any reconciliation the first time the backup bucket/entry is deployed.
That's because we are using an Informer created by the SharedInformerFactory here for receiving watch events, but we are actually using a cached runtime client for retrieving the object in the reconciler which uses a separate watch. This in turn means, that the cached runtime client might not have received the add event for a backup bucket/entry, while the Informer already has received an event and enqueued the object for reconciliation. The cached runtime client the gets a NotFound error and the reconciler exits early.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:
In general, we should always use the same informers for registering event handlers which we also use in our controllers to retrieve the object in the beginning of the reconciliation. For example an Informer from the SharedInformerFactory and the respective Lister or an Informer from the controller-runtime cache and the cached runtime client.
I will do this refactoring in a separate PR, as it requires to not invalidate the Garden ClientSet in any case (also for version changes) which I will have to tackle first.
/cc @timuthy

Release note:

A bug has been fixed, that caused that newly created `BackupBuckets`/`BackupEntries` where not reconciled by the `gardenlet` immediately when the `CachedRuntimeClients` feature gate was enabled.

[timebertt]

Probably we should cherry-pick this to release-v1.7.

[timuthy]

/lgtm