Read project_id from credentials (rebase of #143)

[sabman]

Summary

• Squashed/rebased Retrieves Firebase Project ID from the provided service credential file #143 onto current master
• Extracts Firebase project_id from service account credentials
• Keeps explicit project_name with deprecation warning
• Preserves keep-alive / http_options behaviour

Original PR: #143 by @xavarius

Test plan

• Specs pass
• FCM.new(credentials_path) works without project id
• Explicit project name still works (with deprecation warning)
• timeout / keep_alive_connections still work as kwargs after credentials