_get_dialects not working for properties

[markus-96]

Describe the bug
Symptoms:

1. Field.describe does not contain the correct db_field_types.
2. CharField returns the wrong SQL_TYPE for oracle

It is only a small bug I think, but I wanted to share my findings.

To Reproduce

import json

from tortoise import Model
from tortoise.fields import CharField


class SomeModel(Model):
    some_field = CharField(max_length=255)


if __name__ == "__main__":
    print(json.dumps(SomeModel.describe(serializable=True)["data_fields"][0]["db_field_types"], indent=2))
    print(SomeModel._meta.fields_map.get('some_field')._get_dialects())
    print(SomeModel._meta.fields_map.get('some_field').get_for_dialect("oracle", "SQL_TYPE"))

outputs:

{
  "": "VARCHAR(255)"
}
{'oracle': {'field': <tortoise.fields.data.CharField object at 0x000001FB0578D370>}}
VARCHAR(255)

Expected behavior

{
  "": "VARCHAR(255)",
  "oracle": "NVARCHAR2(255)"
}
{'oracle': {'SQL_TYPE': 'NVARCHAR2(255)', 'field': <tortoise.fields.data.CharField object at 0x000001FB0578D370>}}
NVARCHAR2(255)

Additional context
Found this during fiddling around with #1853 ...