[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. Note that this requires the equivalent changes to be submitted in the C++ repo first.

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Testing

Describe how you've tested these changes.

https://github.com/firebase/firebase-unity-sdk/actions/runs/30405465237
And running the tests locally with both the old and new methods enabled.

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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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[gemini-code-assist]

Code Review

This pull request introduces new Firebase Messaging registration methods using Installation IDs (FIDs) and deprecates the older token-based methods across the SDK, SWIG wrappers, and sample applications. The review feedback highlights critical safety improvements, specifically adding null checks for installation_id in the SWIG callbacks to prevent potential crashes, and adjusting the automated test logic to gracefully handle the new registration flow and avoid test flakiness.