Add `configure` automation for `Secrets.swift`

[spencertransier]

In #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.