Need a way to set mutable_content and content_available header fields

[mazz]

Is your feature request related to a problem? Please describe.

For 2.0.0.rc.2, there is no way to set the mutable_content and content_available header fields.

This was present in the LegacyFCM through:

      |> Pigeon.LegacyFCM.Notification.put_mutable_content(true)
      |> Pigeon.LegacyFCM.Notification.put_content_available(true)

Describe the solution you'd like
Add an api to Pigeon.FCM.Notification for put_mutable_content() and put_content_available()

[trrstn]

Is there a work around to set content_available to true?