tts: add speaker file support

[dm4]

• Added support for TTS speaker files, including a new command-line option --tts-speaker-file to specify the file path.
• Implemented JSON handling in tts.cpp to load and parse speaker data, enhancing audio generation capabilities.

[ngxson]

@edwko Could you please have a look on this PR?

[edwko]

@ngxson @dm4 Looks good! Just a couple of thoughts, this would handle only v0.2 it might make sense to do this more dynamically, maybe add versioning logic similar to this PR #11287

Maybe get version from common_get_builtin_chat_template, or I could add more metadata to the speaker files (like a version fields) to construct the prompt based on the specific version.

// Something like this:

double get_speaker_version(json speaker) {
    if (speaker.contains("version")) {
        return speaker["version"].get<double>();
    } 
    // Also could get version from model itself
    // if (common_get_builtin_chat_template(model) == "outetts-0.3") {
    //     return 0.3;
    // }
    return 0.2;
}

static std::string audio_text_from_speaker(json speaker) {
    std::string audio_text = "<|text_start|>";
    double version = get_speaker_version(speaker);
    
    if (version <= 0.3) {
        std::string separator = (version == 0.3) ? "<|space|>" : "<|text_sep|>";
        for (const auto &word : speaker["words"])
            audio_text += word["word"].get<std::string>() + separator;
    }
    else if (version > 0.3) {
        // Future version support could be added here
    }

    return audio_text;
}

// static std::string audio_data_from_speaker(json speaker) would also need some adjustments to support different versions.