Create publisher entity

[knjk04]

Create a many-to-many relationship between Book and a new Publisher entity

Fields:

• Publisher name. This can be a String (we can remove our existing enum). This should be unique
• Books (non-null)
• About (nullable)

We should be able to find all of the books published by that publisher (derived query with JPQL).

Additional context

A new migration is needed for Flyway