Add `default` cluster to the audit log

[andrioni]

What version of Materialize are you using?

v0.26.1-dev (0a77973af)

How did you install Materialize?

Built from source

What is the issue?

Whenever a new environment is created, a default cluster with a single replica is also created, but this cluster creation is not included in the audit logs right now.

Relevant log output

No response