feat: add mixin for int enum field custom

[waketzheng]

Description

As #966 said, IntEnumField inherit from SmallIntField, so it can not use a enum_type with value > 32767

Motivation and Context

This PR add a IntEnumFieldMixin to make it easy to custom a new IntEnumField:

from typing import Type, cast
from tortoise.fields.data import IntEnumFieldMixin, IntEnumType, IntField

class IntEnumInstance(IntEnumFieldMixin, IntField):
    pass

def IntEnumField(enum_type: Type[IntEnumType], **kwargs) -> IntEnumType:
    return cast(IntEnumType, IntEnumInstance(enum_type, **kwargs))

How Has This Been Tested?

make ci

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have added the changelog accordingly.
• [x] I have read the CONTRIBUTING document.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.

[coveralls]

Pull Request Test Coverage Report for Build 11994687751

Details

• 37 of 37 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.002%) to 89.507%

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Totals
Change from base Build 11994314143:	-0.002%
Covered Lines:	6234
Relevant Lines:	6856

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💛 - Coveralls