Add support for option_message_padding
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We add prototypical support for the `option_message_padding` feature while the BOLTs PR is still underway.
When both parties signal support for `option_message_padding`, we pad any sent messages to a fixed size to improve privacy in the face of an adversary monitoring network traffic. To this end we utilize an optional TLV-stream extension with an odd field number of `u64::max_value()` that simply will be discarded by the counterparty. The padding threshold is chosen to fit even the largest standard Lightning messages (UpdateAddHtlc) whith some leeway to guarantee package size uniformity even when some of the optional fields are set. Note that even without padding we surpassed the standard Ethernet MTU of 1500 bytes for `UpdateAddHtlc` messages, so fitting the packets into exactly 1500 bytes is a futile endeavor. Furthermore note that any messages above that threshold size will still stand out in monitored network traffic. Lastly, we opt to *not* apply padding for any custom messages, as they might not be set up to handle the optional TLV extension.
While it shouldn't really make any difference for the Noise protocol, we here avoid taking any chances w.r.t. known plaintext attacks and opt to randomize the padding data.
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We add support for
option_message_padding(feature ??) as proposed in lightning/bolts#1304.