Add an -all_load linker flag to all example apps (#3315)

* Add an -all_load linker flag to all example apps

This fixes macOS and tvOS builds such that they no longer need to
explicitly link against the built FirebaseFirestore framework, which is
good because this wasn't working right: changes to the framework would
take two builds to get tests to actually see them.

The mechanism here is to amend the .xcconfig files that CocoaPods plugs
into the project, just after it generates them. This avoids the need to
actually modify the project.xcodeproj file, which reduces the potential
for conflicts.

Going forward all manual modifications to the project's configuration
will be handled this way to avoid needing special project configuration.

This also moves us incrementally closer to be being able to use test
specs and cocoapods-generate. There's too much custom configuration yet
(and several other blockers) but figuring out what's required to build
will make it such that we can eventually transition.

* xcodeproj: Remove -all_load and -framework flags

-all_load is now being supplied by sync_project.rb.

Remove manually specified -framework flags from the project file. The
-framework flags are actually coming from CocoaPods, but we copied the
others when manually linking against FirebaseFirestore. Now that that's
no longer required these can go away.

* Add a test-only mode to sync_project.rb

* Re-add CodableIntegrationTests.swift

A previous commit merged this incorrectly.

Author: wilhuff