The library is not Mac/iThing specific. UniFFI supports other languages that work on all platforms. Furthermore, Swift actually supports Linux and we might want the SDK to support that platform one day.

It therefore seems reasonable to avoid unknowingly adding anything that breaks Linux support in this library. Hence we should also test the Rust code on Windows and Ubuntu.

We use `ProcessInfo`, which is part of `Foundation`, to read env vars in the package spec.

The package looks to be automatically imported in newer versions of Swift/macOS, but to support older ones we need to do it explicitly.

Rename generated framework to `ConcordiumWalletCryptoUniffi.xcframework` to match the (similarly renamed) binary target in `Package.swift`. We cannot just rename the target to match the existing generated file because that name (`ConcordiumWalletCrypto`) is already used by the library product target.

This is a requirement that apparently has been lifted in newer versions of Swift/macOS, but to support older versions, we follow it.

Also updated changelog.

For the purpose of verifying that the library builds on CI, there's no point in building the binary framework for all platforms, just for the build platform itself.

We take into account that some ("older") runners are Intel macs while others ("newer") run on Apple Silicon.