gguf : add special tokens metadata for FIM/Infill (#6689)

This commit adds special token metadata for Fill-In-the-Middle
(FIM)/Infill to the GGUF model.

The motivation for this is that currently there is support for CodeLlama
but other models exist now like CodeGemma, but the different models use
different token ids for the special tokens and this commit allows for
supporting multiple models.

Signed-off-by: Daniel Bevenius <daniel.bevenius@gmail.com>