Support PiTR with BR and TiCDC

Feature Request

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

After the first version of PiTR https://github.com/pingcap/br/issues/325 finished.
It's is better to support it with operator. it will improve the availability of TiDB cluster.

Describe the feature you'd like:

Basically operator need integrate several functions in BR and TiCDC to implement PiTR, and according to current conclusion,
operator only need interact with BR to do PiTR. such as

1. BR full/differential backup/restore
2. BR log backup/restore(BR use TiCDC client to generate task)
3. BR log undo --ts range
4. BR gen differential-backup --differential-backup --log
5. BR checksum

Describe alternatives you've considered:

discussed with @DanielZhangQD , we will use CRD to integrate PITR in tidb-operator, plan in Q4

Teachability, Documentation, Adoption, Migration Strategy:

Workload estimation(person-day)

• Design+Develop: 20
• QA: 5
• Document: 5

Start by reading BR issue #325 and the proposed CRD integration plan for tidb-operator. Map how the operator would expose BR full, differential, log backup/restore, log undo, differential-backup, and checksum operations with TiCDC involved; done means the PiTR workflow is integrated through the operator with the planned QA and documentation coverage.