C public API: Rename FastConfig to SingleRowPredictor

[Ten0]

Fast Config in the C API is poorly named, in that it's actually specific to single row prediction, and doesn't only contain config.
-> the items named around FastConfig should be renamed to be made named around SingleRowPredictor, because that is what they are actually relating to.
We should make this change in the next release which may contain breaking changes.

https://github.com/Ten0/LightGBM/blob/063008a7e5caafa1c5a0171bf5cad10bda3e044c/src/c_api.cpp#L2122-L2132
(link to be made pointing to master once #6024 is merged)

[jameslamb]

FastConfig is poorly named

Can you suggest some names that you think are more informative or clearer?

[Ten0]

Can you suggest some names that you think are more informative or clearer?

Whoops, looks like I forgot this this time. 😅
Thanks, updated message above.