Add modular headers support to Objective-C Firebase pods for static framework compatibility

This commit adds DEFINES_MODULE = YES to the pod_target_xcconfig section of
FirebaseCore, FirebaseInstallations, and FirebaseCoreExtension podspecs.

## Problem
Swift-based Firebase pods (FirebaseCoreInternal, FirebaseCrashlytics, FirebaseSessions)
depend on Objective-C Firebase pods. When using static frameworks via
'use_frameworks! :linkage => :static', Swift cannot import Objective-C dependencies
unless those dependencies define module maps.

This prevents React Native (New Architecture), Expo, and other frameworks from using
Firebase with static linking, resulting in errors like:
- "The Swift pod X depends upon Y, which does not define modules"
- "include of non-modular header inside framework module"

## Solution
Enable module map generation by adding 'DEFINES_MODULE' => 'YES' to the
pod_target_xcconfig in:
- FirebaseCore.podspec
- FirebaseInstallations.podspec
- FirebaseCoreExtension.podspec (new section added)

When DEFINES_MODULE is enabled, CocoaPods automatically generates module.modulemap
files, allowing Swift code to properly import these Objective-C dependencies in
static framework builds.

## Impact
- Enables static framework usage for React Native, Expo, and other frameworks
- Maintains full backward compatibility with existing projects
- No breaking changes - dynamic frameworks continue to work as before
- Aligns with modern iOS build practices and CocoaPods best practices

## Testing
All modified podspecs pass validation:
- pod spec lint FirebaseCore.podspec --quick --allow-warnings
- pod spec lint FirebaseInstallations.podspec --quick --allow-warnings
- pod spec lint FirebaseCoreExtension.podspec --quick --allow-warnings

## Related Issues
Resolves #15249
Related to:
- invertase/react-native-firebase#6933
- invertase/react-native-firebase#6332
- expo/expo#39607

## Note
External dependencies (GoogleUtilities, GoogleDataTransport, nanopb) are maintained
in separate repositories and would benefit from similar changes for complete resolution.

Author: MichaelBernhardt

FirebaseCore.podspec

@@ -56,6 +56,7 @@ Firebase Core includes FIRApp and FIROptions which provide central configuration
   s.dependency 'FirebaseCoreInternal', '~> 12.7.0'
 
   s.pod_target_xcconfig = {
+    'DEFINES_MODULE' => 'YES',
     'GCC_PREPROCESSOR_DEFINITIONS' => 'Firebase_VERSION=' + s.version.to_s,
     'HEADER_SEARCH_PATHS' => '"${PODS_TARGET_SRCROOT}"',
     'OTHER_CFLAGS' => '-fno-autolink'

FirebaseCoreExtension.podspec

@@ -34,5 +34,9 @@ Pod::Spec.new do |s|
       "#{s.module_name}_Privacy" => 'FirebaseCore/Extension/Resources/PrivacyInfo.xcprivacy'
     }
 
+    s.pod_target_xcconfig = {
+      'DEFINES_MODULE' => 'YES'
+    }
+
     s.dependency 'FirebaseCore', '~> 12.7.0'
   end

FirebaseInstallations.podspec

@@ -52,6 +52,7 @@ Pod::Spec.new do |s|
 
   preprocessor_definitions = ''
   s.pod_target_xcconfig = {
+    'DEFINES_MODULE' => 'YES',
     'GCC_PREPROCESSOR_DEFINITIONS' => preprocessor_definitions,
     'HEADER_SEARCH_PATHS' => '"${PODS_TARGET_SRCROOT}"'
   }