Feature request - C API - support raw data as input for in-place prediction

[tlkhm]

In-place prediction is supposed to be fast and efficient according to https://xgboost.readthedocs.io/en/stable/prediction.html#in-place-prediction

C API only supports JSON string as input for in-place prediction which requires conversion from raw data to char array which later is converted back to raw data. It would be more efficient to support raw data without JSON conversion. In my case, the input is a simple 1D float array.