Support Notification Interruption Level in Cloud Messaging in a better way

As you may know, Apple introduced `UNNotificationInterruptionLevel` last year that allow us to set the interruption level of a notification to `active`, `passive`, `timeSensitive` and `critical`.
At the moment, I was able to make it work with Firebase by adding a key `interruption-level` to the custom data and by extracting it from the `request.content.userInfo` like the following:
```
import UserNotifications
import FirebaseMessaging

class NotificationService: UNNotificationServiceExtension {

var contentHandler: ((UNNotificationContent) -> Void)?
var bestAttemptContent: UNMutableNotificationContent?

override func didReceive(_ request: UNNotificationRequest, withContentHandler contentHandler: @escaping (UNNotificationContent) -> Void) {
self.contentHandler = contentHandler
bestAttemptContent = request.content
.mutableCopy() as? UNMutableNotificationContent
guard let bestAttemptContent = bestAttemptContent else { return }

// Adding Notification Interruption Level support
if let rawInterruptionLevel = bestAttemptContent.userInfo["interruption-level"] as? String {
var interruptionLevel: UNNotificationInterruptionLevel = .passive
if rawInterruptionLevel == "1" || rawInterruptionLevel == "active" {
interruptionLevel = .active
} else if rawInterruptionLevel == "2" || rawInterruptionLevel == "timeSensitive" {
interruptionLevel = .timeSensitive
} else if rawInterruptionLevel == "3" || rawInterruptionLevel == "critical" {
interruptionLevel = .critical
}
bestAttemptContent.interruptionLevel = interruptionLevel
}

FIRMessagingExtensionHelper().populateNotificationContent(
bestAttemptContent,
withContentHandler: contentHandler)
}

```
I think this could be improved by adding and Interruption level picker in the "Additional options (optional)" instead of having to add it to the custom data like this:

A picker like for "Sound" or "Apple badge" would be ideal and make marketing people who are creating campaigns more happy ;-).

On the iOS SDK side, I noticed the `FIRMessagingExtensionHelper().populateNotificationContent(...` and I truly believe that it could be improved.
https://github.com/firebase/firebase-ios-sdk/blob/3512851cf669f2173f04b415e64a1ded020595a2/FirebaseMessaging/Sources/FIRMessagingExtensionHelper.m#L108

In fact, I believe that following code snippets logic should be part of it:
````
// Adding Notification Interruption Level support
if let rawInterruptionLevel = bestAttemptContent.userInfo["interruption-level"] as? String {
var interruptionLevel: UNNotificationInterruptionLevel = .passive
if rawInterruptionLevel == "1" || rawInterruptionLevel == "active" {
interruptionLevel = .active
} else if rawInterruptionLevel == "2" || rawInterruptionLevel == "timeSensitive" {
interruptionLevel = .timeSensitive
} else if rawInterruptionLevel == "3" || rawInterruptionLevel == "critical" {
interruptionLevel = .critical
}
bestAttemptContent.interruptionLevel = interruptionLevel
}
````
Again, this is just a temporary solution and you could restrict values if it was part of the SDK.
Finally, that part of the SDK is still in Objective-C. Could be nice to have it in Swift (if it's not already in your roadmap scope ^^).

Hoping that this helps other devs who wants to implement this feature or that it contributes to improve the SDK.

Anyhow,
Thank you,
Marc