Manage ios-firebase-sdk with SwiftPM

[uhooi]

References

• https://github.com/firebase/firebase-ios-sdk/blob/master/SwiftPackageManager.md
• https://github.com/firebase/firebase-ios-sdk/issues/6472
• https://github.com/firebase/firebase-ios-sdk/issues/3136
• https://fromatom.hatenablog.com/entry/2020/09/28/224809
• https://qiita.com/temoki/items/da91bffe5a347103b0ca
• https://twitter.com/shimastriper/status/1345724104888860674?s=20

[github-actions[bot]]

👋 Thanks for reporting! Please PR :)