Need a way to be able to test Message Mutators in isolation

[awright18]

Describe the feature.

Is your feature related to a problem? Please describe.

This is related to a post on discuss.particular.net

Describe the requested feature

Create some way to provide a MutateOutgoingTransportMessageContext to a message mutator so that it can be evaluated in a test.

Describe alternatives you've considered

I've created a custom factory to do this, but it would be nice if there was something in the main framework to handle this.

Additional Context

No response