Access username as a field when using `forward` Input plugin

[semekh]

Username of an authenticated user should be available as part of the log processing workflow, so that the process can be customized based on it.

Use case: I'd like to have a log aggregator server that accepts log records from multiple users. And each user should only be able to send specific types of logs (e.g. say, only records with tag=${username}.* are allowed)