text: Title: Add a `pretty` option when dumping fixtures
  Body: When we dump fixtures:
  
  ```
  piccolo fixtures dump my_app
  ```
  
  It prints out the JSON, but it isn't formatted nicely. It would be good to have a `pretty` option:
  
  ```
  piccolo fixtures dump my_app --pretty
  ```
  
  Here's the relevant code:
  
  https://github.com/piccolo-orm/piccolo/blob/74ea10d427464b05d4a0caba05ce317582d74203/piccolo/apps/fixtures/commands/dump.py#L111-L128
  
  The simplest solution is to replace this line:
  
  https://github.com/piccolo-orm/piccolo/blob/74ea10d427464b05d4a0caba05ce317582d74203/piccolo/apps/fixtures/commands/dump.py#L128
  
  With this:
  
  ```python
  if pretty:
      print(json.dumps(json.loads(json_string), indent=4))
  else:
      print(json_string)
  ```
  
  Though there may be a better way of doing it. We use Pydantic to create the JSON string, and there may be a way of getting it to do a pretty output, but I'm not sure.