Add IOS_FCM_ENABLED variable to allow disabling FCM functionality on iOS

[aleh]

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Documentation changes
• Other... Please describe:

PR Checklist

Please check your PR fulfills the following requirements:

New features/enhancements:

• Exhaustive testing has been carried out for the new functionality
• Regression testing has been carried out to ensure no existing functionality is adversely affected
• Documentation has been added / updated
• The example project has been update to validate/demonstrate the new functionality.

What is the purpose of this PR?

There is a conflict on iOS in case push notifications are handled by another plugin (Airship), i.e. when FCM functionality of the plugin is not used. Even with FIREBASE_FCM_AUTOINIT_ENABLED set to false the plugin still would set its delegate for UNUserNotificationCenter however preventing regular flow of notifications. This PR is allows to disable FCM on iOS via IOS_FCM_ENABLED variable.

Does this PR introduce a breaking change?

• Yes
• No

What testing has been done on the changes in the PR?

Checked in an isolated example and used in a production app.

What testing has been done on existing functionality?

Other information

[aleh]

Thanks!