Support PITR restore using --start-ts only

Feature Request

Is your feature request related to a problem? Please describe:

Currently TiDB Operator's PITR required a complete data backup as the starting point, i.e. the Restore CR required a valid spec.pitrFullBackupStorageProvider.

---
apiVersion: pingcap.com/v1alpha1
kind: Restore
spec:
  restoreMode: pitr
  pitrFullBackupStorageProvider:  # this item is required
    s3:
      ...
  ...

This is statically checked in

tidb-operator/cmd/backup-manager/app/restore/restore.go

Lines 96 to 101 in ab875aa

    
           if fullBackupArgs, err := pkgutil.GenStorageArgsForFlag(restore.Spec.PitrFullBackupStorageProvider, "full-backup-storage"); err != nil { 
        
           	return err 
        
           } else { 
        
           	// parse full backup path 
        
           	args = append(args, fullBackupArgs...) 
        
           }

Describe the feature you'd like:

In our scenario, the "starting point" is not a single Full backup, but also followed by a series of Incremental backups. The kernel br command supports this PITR scenario via the --start-ts option:

br restore full -s 's3://bucket/full-backup/2024/01'
br restore full -s 's3://bucket/incr-backup/2024/01/01'
br restore full -s 's3://bucket/incr-backup/2024/01/02'
br restore full -s 's3://bucket/incr-backup/2024/01/03'
br restore point -s 's3://bucket/log-backup' --start-ts=446748741183209472 --restored-ts='2024-01-03 13:15:00'

We would like the Restore CR to expose the --start-ts option, making the validation required either --start-ts or --full-backup-storage (but not both).

---
apiVersion: pingcap.com/v1alpha1
kind: Restore
spec:
  restoreMode: pitr
  pitrStartTs: '446748741183209472'   # <- new option
  ...

Describe alternatives you've considered:

Rather than exposing --start-ts, we can make the Restore CR perform the (Full + Incremental × N) steps like

---
apiVersion: pingcap.com/v1alpha1
kind: Restore
spec:
  restoreMode: pitr
  pitrFullBackupStorageProvider: ...
  pitrIncrementalBackupStorageProviders:
  - s3: ...
  - s3: ...

But this is considerably more complex and is not suitable for our need (which we have performed the snapshot restore by other means already)

Teachability, Documentation, Adoption, Migration Strategy:

Update https://docs.pingcap.com/tidb-in-kubernetes/stable/restore-from-aws-s3-using-br#point-in-time-recovery (and the equivalent page for other environments if any) to include the new option.

	if fullBackupArgs, err := pkgutil.GenStorageArgsForFlag(restore.Spec.PitrFullBackupStorageProvider, "full-backup-storage"); err != nil {
	return err
	} else {
	// parse full backup path
	args = append(args, fullBackupArgs...)
	}