Support await tortoise.contrib.fastapi.RegisterTortoise

[waketzheng]

Description

1. Run make style
2. Update changelog
3. Update RegisterTortoise class document
4. Use typing.Self to improve type hint
5. Support await for RegisterTortoise and add testcase for this feature

Motivation and Context

Sometimes, I want to use await RegiterTortoise(...) instead of async with RegiterTortoise(...)

How Has This Been Tested?

make ci

Checklist:

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