server: allow filtering llama server response fields

[nvrxq]

This adds a new feature. Allow Filtering LLama Server Response Fields from #10819.
Usage:
Curl:

curl --request POST \
    --url http://localhost:8080/completion \
    --header "Content-Type: application/json" \
    --data '{"prompt": "Building a website can be done in 10 simple steps:","n_predict": 128, "requested_fields": ["content", "generation_settings/n_predict"]}'

Response:

{"content":" 1. Choose a domain name...","generation_settings/n_predict":128}

Retrievals from multiple nested lists are also supported, example:
data  = {"key1" : {"key2": 3}} requested_fields = ["key1/key2"] - > {"key1/key2": 3}.

If response does not contain a path, we will return empty json for path.