Expose the IProcessingSourceTableProvider from ProcessingSource

[jmaxson-ms]

Describe the bug A custom table provider has to be passed into the constructor for ProcessingSource, not providing much of an opportunity for initializing the object. If the table provider was exposed as a protected property from the ProcessingSource, this would alleviate the issue without being a breaking change.

To Reproduce Steps to reproduce the behavior:

1. Create a plugin that uses feature X/does Y
2. Load the plugin with WPA/the Engine
3. Perform actions ...

Expected behavior A clear and concise description of what you expected to happen.

Actual behavior A clear and concise description of what actually happens.

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