[Firestore] Add warning when building Firestore's binary SPM distro for visionOS

[ncooke3]

The proposed consists of two parts:

1. Change the Package.swift to only build Firestore's binary targets on platforms that support the binary distribution (e.g. !visionOS)
   • This shifts the problem from the package loading phase to the compilation phase.
2. Add a warning in the source wrapper target to issue a warning only on visionOS and only when it is detected that the client is using the binary distro*.
   • *: We can determine this by configuring the wrapper target in the binary distro code path to define a preprocessor directive.

[google-oss-bot]

Coverage Report ¹

Affected Products

• FirebaseFirestore-iOS-FirebaseFirestoreInternal.framework

  Overall coverage changed from 88.10% (795c800) to 88.17% (a36fd8b) by +0.06%.

  Filename	Base (795c800)	Merge (a36fd8b)	Diff
  exception.cc	23.68%	84.21%	+60.53%
  leveldb_key.cc	98.82%	98.33%	-0.49%
  ordered_code.cc	94.39%	93.90%	-0.49%

Test Logs

• Base (795c800): https://github.com/firebase/firebase-ios-sdk/actions/runs/7506782846
• Merge (a36fd8b): https://github.com/firebase/firebase-ios-sdk/actions/runs/7545006298

1. https://storage.googleapis.com/firebase-sdk-metric-reports/0OlvLiNKHk.html

[andrewheard]

Thanks, Nick!