chore(firebase_core): create a swift package of shared iOS source code for all plugins

[russellwheatley]

Description

• created a firebase core shared package in the root of FlutterFire repo for source code that is shared between all plugins. this will be pulled in by all plugins including firebase core. Symlinked code that is shared into Sources/ directory in the root of repo.
• created a script that tags ${firebase-core-version}-firebase-core-swift so we can keep it up-to-date.

I've tested this with my own FlutterFire fork so I think once I tag main branch by running the [generate_tag_spm_firebase_core.dart manually, I should be able to use it in the SPM firestore PR, and get that ready for review.

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