Expand SPM infra

[ncooke3]

Capturing future work section from firebase/firebase-ios-sdk#14818

Future work

• Update tooling to update an Xcodeproj from pointing to a firebase-ios-sdk release, as well as the main branch.
• Add other Firebase products with SPM quickstarts and delete associated Legacy quickstart CocoaPods projects
• [Low priority] Add run/test support to the quickstarts in addition to build