export more core types and test helper utils

[kevinwhereby]

Description

I'm starting to write tests for the transcription-client and felt these could come in handy.

We interact directly when using the /core export, so exporting a helper to create a mock store is very useful
The participant types are also useful

Testing

this is just exports, so as long as you can import them we are golden

Checklist

• My code follows the project's coding standards.
• I have written unit tests (if applicable).
• I have updated the documentation (if applicable).
• By submitting this pull request, I confirm that my contribution is made
  under the terms of the MIT license.