fix: Add id column to association tables and restore unique constraints

[fregataa]

follow-up #576

• Add id column to association_groups_users, sgroups_for_domains, sgroups_for_groups, sgroups_for_keypairs tables
• Restore incorrectly dropped unique constraints from association_groups_users, sgroups_for_domains, sgroups_for_groups, sgroups_for_keypairs tables (related commit code, refactor: Split the session & kernel tables with ORM migration #576)

Checklist: (if applicable)

• Milestone metadata specifying the target backport version
• Mention to the original issue
• Installer updates including:
  • Fixtures for db schema changes
  • New mandatory config options
• Update of end-to-end CLI integration tests in ai.backend.test
• API server-client counterparts (e.g., manager API -> client SDK)
• Test case(s) to:
  • Demonstrate the difference of before/after
  • Demonstrate the flow of abstract/conceptual models with a concrete implementation

[achimnol]

After applying this commit, alembic upgrade head gives:

sqlalchemy.dialects.postgresql.asyncpg.AsyncAdapt_asyncpg_dbapi.ProgrammingError: <class 'asyncpg.exceptions.InvalidTableDefinitionError'>: multiple primary keys for table "association_groups_users" are not allowed

Please check out this! @fregataa