[audio_streamer] Add Swift Package Manager (SPM) support #1299

`audio_streamer` currently triggers a warning when building for iOS with recent Flutter versions:

The following plugins do not support Swift Package Manager for ios: audio_streamer. This will become an error in a future version of Flutter.

I see that `light`, `pedometer`, and `screen_state` already received SPM support in this repo via #1288, but `audio_streamer` appears to have been left out. Could SPM support be added here too?

For reference, there's a tool that automates most of the SPM migration for Flutter plugins: https://pub.dev/packages/spm_migrator

Happy to test a PR if that's helpful. Thanks for maintaining this package!