feat: add attributes column to data_source table

[victorgarcia98]

• Add attributes column to the table data_source to store DataGenerator configuration. Type: JSON.
• Add attributes_hash column to the table data_source to store a SHA256 hash of the attributes column. Type LargeBinary(length=256), in PostgreSQL this is represented as a BYTEA type.
• Add sensorsbackref to the DataSource to allow to get the sensors that use a DataSource (in a Timed Belief).
• Implementation of the methods get_attributes, has_attributes and set_attribute.
• Update the function get_or_create_source to take the attributes column into account.

[Flix6x]

Nice to see you are writing your own db migration. And thanks for dividing your work in these smaller PRs.

I'm looking forward to seeing how this will tie in with persisting reporter configurations. I have some small comments for now, and the changelog should also get a db upgrade notification for 0.15.

[Flix6x]

Maybe a changelog entry for this could note that the DataSource table now allows storing arbitrary attributes as a JSON (without content validation), similar to the Sensor and GenericAsset tables.

[Flix6x]

🎉 Just still add the db upgrade warning in the changelog (you can copy the warning from earlier versions in the changelog).