Allow 'fetch_related', if given no arguments, to fetch every relationship.

Simple feature request that I would be willing to try to open PR for if idea is accepted. If fetch_related is not given any arguments it should fetch all of the relationships tied with that model.

from tortoise import models

class First(models.Model): ...
class Second(models.Model): ...

class Model(models.Model):
    first = fields.ForeignKeyField("First", on_delete=fields.SET_NULL)
    second = fields.ForeignKeyField("Second", on_delete=fields.SET_NULL)

Model.fetch_related()
# The above function call would behave as the function call below.
Model.fetch_related("first", "second")

Some might argue this is too implicit, and I would not argue. Perhaps a better option is a new function, fetch_all_related() that does the equivalent.

Model.fetch_all_related()

This would also include modifications to prefetch_related - whether with no arguments, or prefetch_all_related.

Start with the fetch_related and prefetch_related entry points and trace how relationship arguments are currently handled. Decide whether no-argument calls or new fetch_all_related and prefetch_all_related methods fit the API, including the implicitness concern. Done means all relationships on the model are fetched consistently and the chosen behavior is covered for both APIs.