feat(ios)!: extract FCM related logic to PushPluginFCM

[erisu]

Description

Closes #285

This PR would replace #285. This PR does not upgrade to 10.24.0 as #285 does but a separate PR would handle the version upgrade.

Extracts all FCM related logic to its own class.

Related Issue

Motivation and Context

Improve separation, maintainability, & encapsulation between FCM and non FCM code.

How Has This Been Tested?

• Build
• Run on Simulator
  • Drop APNs payloads
• Run on Device
  • Pushed notification through FCM console

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.