Introduce API V2

[iosmanthus]

This pull request enables the tikv-service to run only in the TiKV cluster in API v2.

• Integration test with a cluster in API v2.

Signed-off-by: iosmanthus [email protected]

[yongman]

toolchain version changes needed?

[iosmanthus]

This pull request depends on feature min_specification which is only enabled in nightly and cargo fmt --all complains some configs are only enabled in nightly. This nightly toolchain version is the same as TiKV.

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