Use Tortoise model instead of raw SQL for managing internal table

[merlinz01]

Main benefit:
The managing of the tortoise_pathway table is cross-database compatible.

Main disadvantage:
Users have to add the tortoise_pathway model to their TORTOISE_ORM configuration.
If I understand correctly this is necessary for aerich, so it's not out of the ordinary.

Side benefit:
The code is more maintainable.

Obscure side benefit:
It would be possible store the migrations data in a separate database from the managed database. That said, I don't know of a situation where that would be useful.

[henadzit]

Thanks for the contribution.

I don't think this is really needed though. All operations with tortoise_migrations are pretty straightforward that can be described with universal SQL. And as you mentioned, this change complicates the configuration.

[merlinz01]

The biggest reason this is needed is that although the SQL itself is universal, the substitution identifiers are different for Postgres vs SQLite vs MySQL. I was getting SQL syntax errors when trying to rollback because the SQL was using ? instead of $1 with Postgres.

Also, while I 100% agree that there isn't really risk of SQL injection with format strings, yet it is still industry best practices to do native substitution, and eliminates one however small area of risk. I could see problems with vulnerability scanners flagging it.