Add setUser Support

## Summary

The errors inbox has the ability to track users impacted, but the PHP agent is not listed as supporting this feature.

https://docs.newrelic.com/docs/errors-inbox/error-users-impacted/

## Desired Behavior

The PHP agent should implement the setUser method described in the error inbox documentation

https://docs.newrelic.com/docs/errors-inbox/error-users-impacted/#set-users