add c api support

[calvin2021y]

please consider add c api header file support, so it can be used as https://github.com/canonical/raft replacement.

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[Licenser]

Been through this in a different place, since openraft is heavily async it will translate extremely poorly to c-land

[calvin2021y]

wrapper it with struct like https://github.com/datastax/cpp-driver future will be a option.

[Licenser]

The problem is that futures in rust and c are very differently implemented you can't just take a rust future and run it in a c asynchronous runtime (or vice versa)

[calvin2021y]

Is is possible to wrapper rust future with C struct and pass into libuv envet loop (or uv_async_t)?