feat: add create_casbin_rule_model() API for Alembic migration support

[Copilot]

• Add create_casbin_rule_model() function for Alembic migration support
• Export function in __init__.py
• Keep changes minimal: short one-line docstring, no new README docs, no new tests

Original prompt

---

This section details on the original issue you should resolve

<issue_title>CasbinRule Table Migration?</issue_title>
<issue_description>What is the intended way of adding CasbinRule table into the existing application?

Having create_table() on an application's startup doesn't seem like a correct way to go [in the long run].

await adapter.create_table()

Considering SQLAlchemy goes with Alembic migration in most cases, are we supposed to auto-generate the migration on our own? I'd seem logical, but casbin_async_sqlalchemy_adapter.adapter::CasbinRule class uses Base metadata object different than my other models.
Or did I miss this aspect in the documentation?</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes CasbinRule Table Migration? #21

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

[hsluoyz]

@copilot don't add so long README text and code comment. Keep it short and simple. Don't add new test code

[Copilot]

@copilot don't add so long README text and code comment. Keep it short and simple. Don't add new test code

Done in 9f9c83c. Removed README documentation and tests, shortened docstring to one line.

[hsluoyz]

@copilot need to add the new create_casbin_rule_model API usage to README (but keep it concise)