fix(@novu/fcm): use message instead of body for data-type notifications

[cherfia]

What change does this PR introduce?

Fixed the bug reported #4639 by including the message property in the data sent by the FCM provider, aligning with Expo documentation.

Why was this change needed?

The notifications were displaying only the titles and not rendering the notification message body.

Other information

While the body property is not required for data-type notifications, it has been retained for backward compatibility to support users who already rely on it.

[p-fernandez]

Thanks for your contribution.
You will need to update the tests based on the change you have done.

[cherfia]

@p-fernandez I have updated fcm.provider.spec.ts to include message property and NX Successfully ran target test for project @novu/fcm

[p-fernandez]

Thank you for your contribution.
🌟

[cherfia]

Thanks! I'm committed to ongoing contributions to Novu as it's integral part of our open-source solution ⭐