feat(daemon): pass model back to ui

[tonyfettes]

Motivation

In order to implement model selector/picker, the UI has to know:

1. What models are currently available: Currently we already have API for this, and UI can even use some pre-configured model list to let the user to pick from.
2. What model are currently used by a task (This PR). Although the UI knows the model when it creates a task by posting to /v1/task, it loses such information if user refreshes the webpage (which causes the UI state to be discarded). Therefore, for the user to be able to select a model, we have to pass the model information during daemon information synchronization events, speicifically daemon.tasks.synchronized.

Features

This PR adds a field to the Task struct used by daemon, storing the model name, and send this information back to client in both:

1. The response of POST /v1/task;
2. The event daemon.tasks.synchronized event send through GET /v1/events.