Add get_next_task endpoint

A `get_next_task` endpoint should be added so that external services can retrieve the first task from the queue. This should return a blueapi call (`TaskRequest` object), not an experiment definition which you can already get by getting the task at position 0.

May be worth waiting for #15 to start this.

## Acceptance Criteria
- A `get_next_task` endpoint is exposed
- This endpoint returns a `TaskRequest` json blob