Allow modifying the ctx of hooks

[stephenh]

• beforeCommit should be able to have a ctx.knex that is the currently-executing transaction
• beforeFlush/etc should be able to have a read-only ctx.knex to prevent writes that aren't during the transaction

Basically idea is to one-time codegen a per-project src/entities/EntityManager.ts that looks like:

export class EntityManager extends JoistEntityManager<MyContext> {
  createReadContext(context: MyContext) MyContext;
  createWriteContext(context: MyContext) MyContext;
}