Add PushDeliveryTarget

[GErP83]

• Use a single PushClient interface for all delivery targets
• Add PushDeliveryTarget for device tokens and topics
• Add unsupportedTarget for providers that do not support a target type
• Improve public API documentation and error descriptions
• Update README examples and explain FCM/APNs targeting differences

[tib]

Why do we introduced another push client protocol?

This is already VERY confusing to me:

• PushClient -> i know i need a client to send push...
• TopicPushClient -> ???

Shouldn't we use an enum to differentiate delivery targets?

enum PushDeliveryTarget {
    case deviceToken(String)
    case topic(String)
}

This way push client code could be:

public protocol PushClient: Sendable {

    func send(
        notification: PushNotification,
        to target: PushDeliveryTarget
    )
}

Also call site would look a lot better:

pushClient.send(
    notification: PushNotification, 
    to: .deviceToken("my-device")
)
pushClient.send(
    notification: PushNotification, 
    to: .topic("my-topic")
)

WDYT @viaszkadi ? 🤔

[tib]

+1: I'm well aware that APNS topic has to be the bundle identifier, and it's completely different what Firebase defines as topic.

Still from a framework perspective, I'd like to have 1 interface to handle things, not multiple abstraction protocols. If APNs has no support for real topics, then we're screwed anyways, so we could simply trigger a fatalError("Topics are not supported using this driver.") when matching that case...

If someone uses the FCM driver then it'll be just fine and we should be able to send to both platforms using topics...