changing formatter priv ctor to public, needed to configure serilog programmatically

[tstojecki]

Fixes #217

Changes

Changing SerilogLogMessageFormatter ctor from private to public to be able to use in config like so

configBuilder.LogMessageFormatter = typeof(SerilogLogMessageFormatter); 

Otherwise this exception is thrown at runtime
Akka.Configuration.ConfigurationException: 'LogMessageFormatter Type must have an empty constructor'

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 SerilogMessageFormatter CTOR is private, can't be instantiated as the default LogMessageFormatter #217
• Changes in public API reviewed, if any.

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[Aaronontheweb]

LGTM

[Aaronontheweb]

LGTM - may as well leave the singleton there for backwards compat for now.