[core split] Service: tikv

This issue tracks splitting the service `tikv` out of `core` as part of the core-split effort.

Tracking issue: #6829

Please follow the steps in the tracking issue strictly:
- keep public API/behavior unchanged;
- move code into the new crate layout described there;
- add/update feature flags and tests as required;
- open a PR and link it to #6829.