Updating EventHubAdapterFactory for extensibility.

[kylemc]

• Making the provider config a protected member that can be accessed in derived factory types
• Making the QueueMapper public so custom implementations can be authored in derived types

[dnfclas]

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[jason-bragg]

Looks good.
Only thing that caught my eye was that the Queue mapper factory is asynchronous. Queue mappers have historically been light weight mapping logic with no external dependencies. I'm curious why an asynchronous factory is needed.
My preference would be to constrain the creation of the mapper to a synchronous call, if possible.

[kylemc]

I'm happy either way. I was just following the pattern of all the other factories in the adapter, but I can make this one synchronous.

[jason-bragg]

Thanks @kylemc