[Option] Whitelist Jetpack IPs for XML RPC

[majick777]

Add an option to automatically whitelist Jetpack IP ranges for use when XML RPC is disabled.
IP Lists can be found at:
https://jetpack.com/ips-v4.json
https://jetpack.com/ips-v4.txt
Also reference: https://jetpack.com/support/how-to-add-jetpack-ips-allowlist/