Add immediateCheckpoint to ObjectStore.spec.configuation.data

Add immediateCheckpoint to ObjectStore.spec.configuation.data:

```
// Control whether the I/O workload for the backup initial checkpoint will
// be limited, according to the `checkpoint_completion_target` setting on
// the PostgreSQL server. If set to true, an immediate checkpoint will be
// used, meaning PostgreSQL will complete the checkpoint as soon as
// possible. `false` by default.
// +optional ImmediateCheckpoint [bool](https://pkg.go.dev/builtin#bool) `json:"immediateCheckpoint,omitempty"`
```
https://pkg.go.dev/github.com/cloudnative-pg/barman-cloud/pkg/api#DataBackupConfiguration

No file or test is named in the issue. Search the Helm chart templates and values for ObjectStore.spec.configuration.data, compare the rendered manifest with the documented DataBackupConfiguration API, and verify that immediateCheckpoint is exposed with the requested optional boolean behavior.

Written by the indexing model from the issue text.