Fixes #33551 - allow searching audits by current_user

A user would like to define a filter that would allow all users to see
their own audits. The only thing that's missing is to define the
scoped_search on Audit model that would accept the current_user value
for audits' user_id and user.

This PR allows to search audits by the following terms

```
user.id = 1
user.id = 100
user.id = current_user

user = admin
user = current_user
```

the syntax is consitent with other places we have similar support (e.g.
the tasks).