Example apps testflight continuous delivery

[JakubJankowski]

In case I won't be able to finish work on this PR before my holidays here is what needs to be done for this PR to be ready for review:
We're mainly waiting for the IT team to create an App Store Connect user that we will use to auth with App Store Connect in the upload_to_testflight step in two Fastfiles: both for the PublisherExampleSwiftUI and Subscriber Example apps.

• We'll need to add a new github Secret - APPLE_APPS_USERNAME, which will contain the email of the App Store Connect user
• We'll need to create a new App Specific Password for this user: https://support.apple.com/en-us/HT204397, and then replace the github secret's value for: APPLE_APPS_APPLICATION_SPECIFIC_PASSWORD with this password. Inn testflight.yml We make a FASTLANE_APPLE_APPLICATION_SPECIFIC_PASSWORD ENV from it, which is automatically detected and used by Fastlane during the upload_to_testflight step.
• Currently, testflight.yml is set to run on pull_request: for testing purposes. Once this whole flow is finally tested and it works fine, it should be set to run only when code is pushed to the main branch, so pull_request: bit needs to be removed.

Additional considerations:

• I'm not sure if we actually have an AMPLIFY_CONFIGURATION secret added (used in testflight.yml). I've used it in a similar way as it's used in the assemble.yml workflow, but that workflow doesn't even create a JSON file or use this created ENV in any way. So we need to double check if everything is fine here.

[lawrence-forooghian]

@QuintinWillison as mentioned in standup, I'm assigning this one to you until the Apple ID is set up. Please assign to me afterwards, thanks!