Create a migration feature for Casbin schema changes

[Sefriol]

Following up the discussion had here, mongoose adapter needs a way to adapt possible schema changes to ensure that previous users of the adapter have a possibility reliable and documented way to upgrade to a new version.

This requires some thinking and design. However the bare minimum should involve:

1. Migrations are done via transactions if available.
2. You should be able to downgrade a migration as well. Incase there are some something goes wrong.
3. Adapter should probably check that each migration is done before being operational.
4. Migrations should be put under tests.

[casbin-bot]

@Gabriel-403 @Zxilly @kingiw @nodece

[Sefriol]

Possible solutions to use:
https://github.com/balmasi/migrate-mongoose
https://github.com/seppevs/migrate-mongo

[Shivansh-yadav13]

hey, @Sefriol I have some idea with migrate-mongo but how is it going to be a feature of the adapter? I am a little confused.

[Sefriol]

Adapter needs to identify that database schema is old and needs to be updated before use. migrate-mongo would take care of the update and then adapter would work as expected.

I think in this particular case it would just update (up()) all documents with p_type to ptype and possibility to downgrade (down()), converting all documents back to p_type. This should be quite reliable with MongoDB transactions.