feat(odin): Allow collecting FCM device token in SDK & demonstrate it in sample app

[odin-posthog]

💡 Motivation and Context

We want to allow customers to send push notifications to their users in workflows. For that, we need to enable the SDK to collect and update the FCM device token that must be used for pushing notifications.

We're not collecting it automatically when initializing so users that don't need this functionality aren't forced to install firebase dependencies, and they have control over which users have their tokens collected (eg. signed-in only)

💚 How did you test it?

Manually in my emulator through the sample app, and with some automated tests

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.
• If there are related changes in the core package, I've already released them, or I'll release them before this one.