Incremental Backup in Raw KV mode

## Feature Request

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

We have used pingcap TiDB to deploy TiKV cluster in Kubernetes. We want to use TiKV only not TiDB, so we have disabled TiDB component.
We want to take backup of our TiKV data so we have setup one pod which has BR tool installed.

Here full backup based on key ranges are working fine but incremental backup function is not supported.

**Describe the feature you'd like:**
We believe incremental backup on Raw KV should be available.

**Describe alternatives you've considered:**

**Teachability, Documentation, Adoption, Migration Strategy:**
We would like to use TiKV for storing a large amount of data so we want incremental backup feature in Raw KV mode. So we do not need to take huge backup each time.