Create a sample for storing secrets in an extension

[zadjii-msft]

Follow up from #102

In #102 we have simple settings that the extension stores in a file. But we should also have simple docs for storing secrets too. Using actual good APIs for storing secrets on Windows (and not just storing them as plaintext)

The Spongebot extension might be a good example

We may also need a set of helper functions for "here's how to store a credential dummy"