llama : allow overrides for tokenizer flags

[ggerganov]

ref #8584 (comment)

The following GGUF keys can now be overrided:

• tokenizer.ggml.add_space_prefix
• tokenizer.ggml.remove_extra_whitespaces

Example:

./llama-cli \
    -m models/phi-3-mini-128k-instruct/ggml-model-f16.gguf \
    -p "<|system|>\nBe concise\!<|end|>\n<|user|>\nWho are you?<|end|>\n<|assistant|>\n" \
    --verbose-prompt -sp -co -s 0 -c 4096 \
    --override-kv tokenizer.ggml.add_space_prefix=bool:false 

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