Added extension point to manage database migrations

[puehringer]

Summary

This PR adds the ability to tightly integrate Alembic migrations into tdp_core applications.

How it works

On startup, the DBMigrationManager retrieves all extension points with the key tdp-sql-database-migration and creates a DBMigration object for each them. This procedure is very similar to how DBConnector are loaded. A DBMigration then can execute all Alembic commands (upgrade, downgrade, history, stamp, ...) with all options within the context of a specific migration.

How to use

The documentation moved to https://wiki.datavisyn.io/tdp/extension-points/db-migration

[thinkh]

Thanks for your implementation and the good documentation! 👍 For me it works as described.

@puehringer Please add an according minor or major release label to the PR.

@lehnerchristian Please have a look if the implementation is fine for you.

[lehnerchristian]

looks good. however I haven't tested it yet

[lehnerchristian]

After the demo yesterday I think we can merge. @thinkh you made some notes as far as I saw. do you want to copy them over?

[thinkh]

I did not take notes, since @puehringer rembembered the to dos.

@puehringer Your turn. 😉

[puehringer]

• Add possibility to configure schema where the alembic_version table is stored
• Check commands which require -arg (especially revision -m "...")

@thinkh Of course I remembered everything 😉

[puehringer]

@thinkh @lehnerchristian The two remaining issues are solved and documented in the wiki. This PR is therefore ready to merge.

[lehnerchristian]

one thing I noticed in the docs: in https://wiki.datavisyn.io/tdp/extension-points/db-migration#initializing-the-migrations-folder you're updating the access rights for the new migration folder

the command you use is sudo chown $USER:$USER -R alembic alembic.ini

however it should be sudo chown $USER:$USER -R migration alembic.ini, shouldn't it?

[puehringer]

@lehnerchristian I changed it to sudo chown $USER:$USER -R migration, as the ini is supplied by tdp_core#dbmigration.ini. Thanks for the hint!