Retention Messaging CLI

[chrisballinger]

This PR is stacked on top of #153 so the diff is a bit noisy until that is merged.

Not sure if this is something that the Apple folks would be interested in merging upstream, but this tool was used to validate that #153 was working properly.

Example usage:

python retention_message.py \
  --key-id "ABCDEFGHIJ" \
  --issuer-id "12345678-1234-1234-1234-123456789012" \
  --bundle-id "com.example.myapp" \
  --p8-file "/path/to/SubscriptionKey_ABCDEFGHIJ.p8" \
  --header "Don't go!" \
  --body "Some real good reason to not cancel your subscription"