Move strictly internal headers from Private to Internal directories

[ncooke3]

Background

Headers within the GDTCORLibrary/Private and GDTCCTLibrary/Private directories are strictly internal to GoogleDataTransport targets. By convention, Private directories denote APIs shared with other frameworks (e.g. across the Firebase monorepo), while Internal directories denote APIs isolated to the framework's own target.

An audit confirms these headers are not used externally in production by any Firebase SDKs. To accurately reflect their scope, they should be migrated to Internal directories.

Action Items

• Create the GDTCCTLibrary/Internal directory (currently missing).
• Move all headers from GDTCORLibrary/Private and GDTCCTLibrary/Private to GDTCORLibrary/Internal and GDTCCTLibrary/Internal.
• Run a global find-and-replace in the GoogleDataTransport codebase to update #import paths to the new Internal locations.
• Coordinated Update: Update the #import path for GDTCORTransport_Private.h in firebase-ios-sdk (specifically inside FirebasePerformance/Tests/Unit/Loggers/FPRGDTLoggerTest.m, which is the only external dependency and is isolated to a test).
• Run GoogleDataTransport unit and integration tests to verify successful compilation.

Notes

• Not a breaking change: This is a purely internal refactor. It does not affect public_header_files or the public CocoaPods/SPM interface.
• Build files (Podspec/Package.swift) rely on folder globs so they shouldn't require structural changes, but verify no explicit Private paths remain hardcoded.