typed_endpoint: creates type for gcm_payload and apns_payload in Remo…

[franciscofleury]

…teServerNotificationPayload

Before this commit RemoteServerNotificationPayload.gcm_payload and RemoteServerNotificationPayload.apns_payload were both Dict[str, Any]. Now a class is used to type these fields. Note that by changing from a Dict to a class gcm_payload and apns_payload properties cannot be acessed with the .get() method anymore since this is a dict-only method.

Fixes:

Screenshots and screen captures:

Self-review checklist

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[alya]

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