Add documentation / ease of use for the `set_provenance` method

[MrCreosote]

The callback server has a set_provenance method intended for testing that apps' generated provenance is correct in kb-sdk tests. There's a couple issues with it:

• It's tricky to use. Since it's not a standard app or server method, a baseclient vs. a specific client has to be used to call the method. We could add a test helper function to do this.
• It's not documented - should add docs to this repo and the kb-sdk docs.