Tracking issue for the HTTP SDK

## Development Task

As https://github.com/pingcap/tidb/issues/35319 mentioned, providing a unified PD HTTP API SDK has become an urgent need. With this means, it could be easier for us to control and observe the use of the PD HTTP API in general.

- [x] Basic implementation. https://github.com/tikv/pd/pull/7304
- [x] Fulfill supported interfaces.
- https://github.com/tikv/pd/pull/7371
- https://github.com/tikv/pd/pull/7397
- https://github.com/tikv/pd/pull/7404
- https://github.com/tikv/pd/pull/7430
- https://github.com/tikv/pd/pull/7449
- https://github.com/tikv/pd/pull/7453
- https://github.com/tikv/pd/pull/7462
- [x] Backoff mechanism.
- https://github.com/tikv/pd/pull/7680
- https://github.com/tikv/pd/pull/7684
- [x] Integrate with other components.
- https://github.com/tikv/client-go/pull/1049
- https://github.com/pingcap/tidb/pull/48276
- https://github.com/pingcap/tidb/pull/48606
- https://github.com/pingcap/tidb/pull/48738
- https://github.com/pingcap/tidb/pull/48858
- https://github.com/pingcap/tidb/pull/49006
- [x] Integration tests.
- https://github.com/tikv/pd/pull/7744
- [ ] pd ctl
- https://github.com/tikv/pd/pull/7605
- https://github.com/tikv/pd/pull/7804
- https://github.com/tikv/pd/pull/7834
- https://github.com/tikv/pd/pull/8212
- https://github.com/tikv/pd/pull/8504
- [x] tools
- https://github.com/tikv/pd/pull/7574
- https://github.com/tikv/pd/pull/8245