server: Add parse_special option to /tokenize endpoint

[IsaacDynamo]

Add parse_special option to /tokenize endpoint.

The parse_special option is explained here #9379.

Tested with:
llama-server.exe --model C:\tools\llm\qwen2.5-coder-1.5b-instruct-q4_k_m.gguf --port 8088

New behavior when parse_special = false:

curl --request POST --url http://localhost:8088/tokenize --header "Content-Type: application/json" 
    --data '{"content": "<|im_start|>Hello World<|im_end|>", "with_pieces": true, "parse_special": false}'
{"tokens":[
    {"id":27,"piece":"<"},
    {"id":91,"piece":"|"},
    {"id":318,"piece":"im"},
    {"id":4906,"piece":"_start"},
    {"id":91,"piece":"|"},
    {"id":29,"piece":">"},
    {"id":9707,"piece":"Hello"},
    {"id":4337,"piece":" World"},
    {"id":27,"piece":"<"},
    {"id":91,"piece":"|"},
    {"id":318,"piece":"im"},
    {"id":6213,"piece":"_end"},
    {"id":91,"piece":"|"},
    {"id":29,"piece":">"}
]}

Old / default behavior (parse_special = true) is unchanged:

curl --request POST --url http://localhost:8088/tokenize --header "Content-Type: application/json" 
    --data '{"content": "<|im_start|>Hello World<|im_end|>", "with_pieces": true}'
{"tokens":[
    {"id":151644,"piece":"<|im_start|>"},
    {"id":9707,"piece":"Hello"},
    {"id":4337,"piece":" World"},
    {"id":151645,"piece":"<|im_end|>"}
]}