Add "swift" flag to firebaseUserAgent (#4448)

* Add "swift-runtime/true" to user agent when Swift detected.

* Use "swift" instead of "swift-runtime"

*  ./scripts/style.sh

* Add comment with the Swift check clarification

* Fix bridging header path

* Fix SwiftObject class name for old Siwft versions.

* Add swift runtime flag with value true or false (to be able to distinguish from a no-value-provided case).

FirebaseCore.podspec

@@ -48,4 +48,12 @@ Firebase Core includes FIRApp and FIROptions which provide central configuration
     unit_tests.dependency 'OCMock'
     unit_tests.resources = 'Example/Core/App/GoogleService-Info.plist'
   end
+
+  s.test_spec 'swift-unit' do |swift_unit_tests|
+    swift_unit_tests.source_files = 'Example/Core/Tests/Swift/**/*.swift',
+                                    'Example/Core/Tests/Swift/**/*.h'
+    swift_unit_tests.pod_target_xcconfig = {
+      'SWIFT_OBJC_BRIDGING_HEADER' => '$(PODS_TARGET_SRCROOT)/Example/Core/Tests/Swift/FirebaseCore-iOS-Unit-swift-unit-Bridging-Header.h'
+    }
+  end
 end