Add SignTools CI to workflow

[crystall1nedev]

Pull request to automatically upload build artifacts for signing on my private SignTools server, allowing GitHub builds to be installed over-the-air.

Keeping a device list here to look on and make sure I got all devices that should be added to my developer account.

• [x] (crystall1nedev) iPad Pro
• [ ] (crystall1nedev) iPhone 6s Plus
• [x] (crystall1nedev) iPhone SE (1st generation)
• [x] (crystall1nedev) iPhone 7
• [x] (crystall1nedev) Apple TV 4K
• [x] (khanhduytran0) Apple TV HD
• [ ] (khanhduytran0) iPhone 6s Plus
• [x] (khanhduytran0) iPhone 8 Plus

Pull request only applies to @khanhduytran0 and I for a few reasons.

• This is my private SignTools instance hosted on a server that isn't completely public.
• My Apple ID is in use, which could lead to spam and getting it locked by users.
• Installing apps from this instance would require that the device's UDID is added to my Apple Developer account.

[crystall1nedev]

Requires a custom build of SignTools to get working as there is currently no way to access SignTools via a command line. Switching to a draft because of this.