Tasks API: Add fields to `codersdk.Task` (required by the UI)

[mafredri]

Update GET /tasks, GET /tasks/{user}/{id}, and codersdk.Task to return additional fields required by the UI.

Goals:

• Response includes the following fields:
  • owner_username
  • template_name
  • template_display_name
  • template_icon
  • workspace_agent_id (nullable, used to fetch agent logs during startup)
  • workspace_agent_lifecycle (nullable, used for knowing agent starting/ready)
  • health (healthy|unhealthy)

Eventual goals:

• Remove workspace_agent_lifecycle in favor of re-thinking the task lifecycle, depends on Tasks API: Design and implement tasks data model in database #948