Full support BJson (BJSONField)

Is your feature request related to a problem? Please describe.
I think it's a good idea to create a separate field - BJSONField, because postgres provides great features with bjson and a lot of queries, which are conveniently handled as an object

Describe the solution you'd like
Model:

class Settings(Model):
    id = fields.BigIntField(index=True)
    data = fields.BJSONField()

Code:

from tortoise.contrib.postgres.objects import bjson_object
...
>>> await Settings.create(data=bjson_object({"a": 1, "b": {"c": "d", "e": True}))
>>> settings = await Settings.all().first()
>>> data = settings.data
>>> data.a
1
>>> data.b.c
'd'
>>> data.b.e
True

And it is also convenient to make a lot of requests for orm, with bjson:

Settings.filter(data={"a":1})
Settings.filter(data__has='a')
Settings.filter(data__has_any=['a', 'b'])
Settings.filter(data__has_all=['a', 'b'])
# Sub/superset of key/value pair testing
Settings.filter(data__contains={'a': 1})
Settings.filter(data__in={'a': 1, 'b': 2})
# Get element/field (compare with json)
Settings.filter(data__get=(2, {'a': 1}))
# Get element/field (compare with scalar, including gt/lt comparisons)
Settings.filter(data__get=(2, 'a'))
Settings.filter(data__get__gt=('a', 1))
# key path traversal, compare with json or scalar.
Settings.filter(data__get=('{a,2}', {'foo': 'bar'}))
Settings.filter(data__get=('{a,2}', 2))
Settings.filter(data__get__lte=('{a,2}', 2))

It would also be nice to implement other postgresql data types

The issue names no files, tests, or entry points. Start by mapping the existing PostgreSQL field and query support against the requested BJSONField behavior and listed filters; done means the supported scope, object access, and query semantics are covered and verified by tests.