gh-138171: Migrate iOS testbed location and add Apple build script (#138176)

Adds tooling to generate and test an iOS XCframework, in a way that will also facilitate
adding other XCframework targets for other Apple platforms (tvOS, watchOS, visionOS and 
even macOS, potentially).

---------
Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>