feat(fcm): Enable fid and deprecate token for Send API

[yvonnep165]

This change deprecates TokenMessage interface, transitioning support to FidMessage interface. A new FidMulticastMessage interface containing only fids: string[] is added. An optional fids is added to the legacy MulticastMessage interface, which is deprecated, to support mixed transition batches.

The multicast helper function sendEachForMulticast is updated to support both target types concurrently up to a combined total of 500. And function overloads are implemented on sendEachForMulticast to support both tokens and fids payloads.

Note for API doc: Hyper-reference link {@link Messaging.sendEachForMulticast} doesn't work in api extraction since there are distinct method declarations due to function overloads. (throwing an ae-unresolved-link error) . So I change it to plain comment text.

[gemini-code-assist]

Code Review

This pull request introduces support for Firebase Installation IDs (FIDs) in the messaging service by adding the FidMessage interface and updating the Message union type. The MulticastMessage interface now supports targeting via fids, and the tokens field has been deprecated and made optional. Feedback includes suggestions to add documentation for the new FidMessage interface, refactor the message construction logic using object destructuring for better maintainability, and improve the grammar of validation error messages. Additionally, the reviewer noted that making tokens optional is a breaking change for TypeScript consumers that should be highlighted in release notes.