Implement End-to-end encryption

Currently, pushover stores messages in plaintext format, but it seems they are encrypted prior to being sent to apple/android pus services. This means that messages can be intercepted should pushover be breached, or where pushover staff act maliciously (to be clear, it’s the possibility that it could happen which is an issue, I am not saying pushover staff are untrustworthy).

I would like the ability to encrypt messages prior to sending them to pushover, that way the above scenarios would be solved provided the secret is not shared with pushover.