Provide some API that wraps etcd API and stores data securely when PD enables encryption at rest

## Feature Request

### Describe your feature request related problem

TiCDC stores metadata in PD's etcd, eg, changefeed information. Unfortunately, some of them contain sensitive data (downstream TiDB username and password). They must be securely stored when PD enables encryption at rest.

### Describe the feature you'd like

PD should provide some API that wraps etcd API and stores data securely.

### Describe alternatives you've considered

- Support encryption at rest for etcd.