Add `configure` automation for `Secrets.swift`

In https://github.com/Automattic/Automattic-Tracks-iOS/pull/166, the PR description mentions this about `Secrets.swift`:

> Create the Shared/Secrets.swift file:
> - For automatticians: copy the file from our mobile secrets repo (under iOS/tracks-demo). You might need to update (git pull) the mobile secrets repo first.
> - For non-automatticians: copy the file Shared/Secrets.example.swift and fill in the missing details
> Feel free to customize your email address :)
> - Note: In a future PR, we'll integrate the configure functionality to automate this, but it's not part of this PR in order to avoid making it any larger.

I found that while figuring out how to fully test a change using the demo app. Let's add automation using `configure` so that the `Secrets.swift` can loaded in easier.