Multiple Configuration / Clients by @apemaia99

[MihaelIsaev]

The original idea is by @apemaia99 in #42:

• Now is possible to load multiple configurations, this is essential in order to send push notifications to different apps.
• Multiple clients are instantiated based on different configuration provided.
• FCM.ID is a convenience struct for identifying a specific client with a specific configuration.

Going to publish it in the next major version. For now, it is available in multiple-clients branch.

Thanks @apemaia99, great job!

[apemaia99]

Thanks @MihaelIsaev, as soon as I have some time you will receive another PR to adapt FCM to the new Vapor concurrency standards with Async Lifecycle.