Add model association override to @auth/sequelize-adapter

[chipgpt]

☕️ Reasoning

I want to be able to customize the associations that are set up between the models when using the @auth/sequelize-adapter. This PR adds an optional override to the SequelizeAdapter options:

new SequelizeAdapter(sequelize, {
  models: {
    ...
  },
  associations: (User, Account, Session) => {
    Account.belongsTo(User, { onDelete: "cascade", foreignKey: "userId", as: 'user' });
    Session.belongsTo(User, { onDelete: "cascade", foreignKey: "userId", as: 'user' });
  }
});

🧢 Checklist

• Documentation
• Tests - There are no tests covering the associations.
• Ready to be merged

🎫 Affected issues

📌 Resources

• Security guidelines
• Contributing guidelines
• Code of conduct
• Contributing to Open Source

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