docs(topics): add external variable providers guide

[vdice]

Adds an External Variable Providers guide.

For the Azure Key Vault example, I wasn't sure how much actual application and Key Vault setup to show, since it is well-documented in the upstream sample app README. Thoughts?

Ref #199

[bacongobbler]

I wasn't sure how much actual application and Key Vault setup to show, since it is well-documented in the upstream sample app README. Thoughts?

My only concern here would be if the upstream sample app README may be moved or updated in the future. I'm not sure that the content inside the documentation would be able to stand on its own if the linked README were to be removed in the future.

However, this looks great. I think it might be worth it to copy those steps into the guide in a follow-up PR (I'd be happy to do that). That way the reader can follow the guide and copy the steps without having to reference back and forth between several documents. This does run the risk of having to update two locations if these steps were to change, however I think this is a more suitable alternative than a potential scenario where a user follows the guide that links to a github page that no longer exists.

[vdice]

@bacongobbler good call. I've update the guide in both Vault and Azure Key Vault sections to be more standalone. Please take a look when convenient.