Support backup and repair task scheduling for v1alpha1.ScyllaDBCluster

[rzetelskik]

Requires:

• Support multiple object storage buckets in E2E test framework #2561
• https://github.com/scylladb/scylla-operator-release/issues/411

Acceptance criteria

• Extend ScyllaDBManagerTask with scheduling tasks for ScyllaDBClusters
  • Includes extending the API to support references to ScyllaDBClusters
  • Includes controllers and finalizers
  • Includes extending validation of references to ScyllaDBClusters
• Add an E2E test for scheduling backup and repair tasks with ScyllaDBClusters