Add custom log level to stream Log stage

[Arkatufus]

Fixes #7127

Changes

Add a new argument to Log() stage to directly specify the log level.

Checklist

For significant changes, please ensure that the following have been completed (delete if not relevant):

• This change follows the Akka.NET API Compatibility Guidelines.
• This change follows the Akka.NET Wire Compatibility Guidelines.
• I have reviewed my own pull request.
• Design discussion issue Akka.Streams: make LogLevel customizable on the .Log method #7127
• Changes in public API reviewed, if any.

[Aaronontheweb]

Has some breaking API changes (additional optional parameters) so this is going to require us to release new Alpakka and Akka.Streams.Kafka plugins