DevX - Automatic/Cloud signing

[pixlwave]

As a developer I want to be able to use automatic/cloud signing when making the release.

This is a requirement for Xcode Cloud, and makes our signing process more secure by removing the need to distribute a certificate and allowing signing privileges to be revoked via App Store Connect. WWDC21-10204 has more information about how Cloud signing works. Ideally we should adopt this for Element iOS too.

App Store Connect user options

Signing for ElementX PR on GitHub can be achieved with additional build flags, however as it is AdHoc this wouldn't necessarily be a requirement to implement.

[stefanceriu]

Automatic signing is already enabled by default and for alpha builds we manually update the settings through a fastlane action here https://github.com/vector-im/element-x-ios/blob/develop/fastlane/Fastfile#L15

[stefanceriu]

Done as part of /issues/81