Reject payloads before they are sent to APNs if they're too large

[TysonAndre]

Request ended with status 413 and responseData: {"reason":"PayloadTooLarge "}

https://developer.apple.com/library/archive/documentation/NetworkingInternet/Conceptual/RemoteNotificationsPG/CreatingtheNotificationPayload.html#//apple_ref/doc/uid/TP40008194-CH10-SW1 indicates that

• For regular remote notifications, the maximum size is 4KB (4096 bytes)
• For Voice over Internet Protocol (VoIP) notifications, the maximum size is 5KB (5120 bytes)

For example, add a default of 4096 bytes, and allow it to be overridden to a larger positive integer for VoIP or to account for future changes to APNs limits

• Buffer.from(string) can be used to convert the raw payload to the bytes to send, and buffer.length gets the length in bytes (instead of codepoints)

If, for example, a client were to attempt to send a JSON payload that was 1 megabyte in size due to failing to truncate part of the input, it would be useful to prevent something that would definitely fail from being sent to APNs, especially if it blocked other requests to APNs or APNs server closes the connection (haven't checked if they currently do that).

(lib/provider.js is what does the final JSON.stringify)

[parse-github-assistant]

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