Add audit logging to lab 7 - secure by design

[altenstedt]

We really want to show a pattern for audit logging in the secure by design lab. It is a very important aspect that you need to consider, and a pattern that we know works would benefit readers.

For example:

await auditService.Log(DomainEvent.ProductRead, product);

One thing to consider is how to deal with the domain object. It can contain secret information that should be removed first. We can do it by mapping to a separate contract class, or by anonymising the domain object. You need to choose when you implement this issue.

[TobiasAhnoff]

This has been implemented in #13