[Messaging] Add new Registration methods

[a-maurice]

Description

Provide details of the change, and generalize the change in the PR title above.

Adds support for the new Messaging registration method, which uses Installation Ids over the old FCM tokens. Note that it is not on by default, and requires setting the appropriate variable in the Android or iOS project.

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Testing

Describe how you've tested these changes. Link any manually triggered Integration tests or CPP binary SDK Packaging Github Action workflows, if applicable.

Running Android and iOS tests locally

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Type of Change

Place an x the applicable box:

• Bug fix. Add the issue # below if applicable.
• New feature. A non-breaking change which adds functionality.
• Other, such as a build process or documentation change.

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Notes

• Bug fixes and feature changes require an update to the Release Notes section of release_build_files/readme.md.
• Read the contribution guidelines CONTRIBUTING.md.
• Changes to the public API require an internal API review. If you'd like to help us make Firebase APIs better, please propose your change in a feature request so that we can discuss it together.

[gemini-code-assist]

Code Review

This pull request introduces a new registration and unregistration mechanism using Firebase Installation IDs (FIDs) to replace the deprecated token-based registration in Firebase Messaging. It adds Register() and Unregister() APIs, introduces OnRegistrationReceived and OnUnregistrationReceived callbacks to the Listener and PollableListener classes, and updates the Android, iOS, and stub implementations accordingly. Two issues were identified in the test code: an unused variable expected_error in integration_test.cc that could trigger compiler warnings, and a logical contradiction in TestUnregistrationClearsCachedRegistration within messaging_test.cc that will cause test failures on the Android desktop stub.

[a-maurice]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces new registration and unregistration methods using Firebase Installation IDs (FIDs) in the Firebase Messaging C++ SDK, deprecating the older token-based registration methods across Android, iOS, and stub implementations. The changes include updates to the C++ API, JNI bindings, Java services, iOS delegates, and flatbuffer schemas, along with comprehensive integration and unit tests. The review feedback recommends adding defensive null checks in MessageReader::ConsumeRegistrationReceived and MessageReader::ConsumeUnregistrationReceived to prevent potential null pointer dereferences and crashes when processing malformed flatbuffer payloads.