Although this is an non-public APIs, but it is already being productized and will be in public SDK some day.

Please add proper doxygen entries for ml-service-ext APIs along with usage examples.

This is required especially for its role when similar API exists in non-ML-Service-Extension API (e.g., _ml_service_extension_request )
https://github.com/nnstreamer/api/blob/018cc9ea83954284e68cd8d20c9207679eac7705/c/src/ml-api-service-extension.h#L57