Investigate whether or not to add random padding via tlv streams to Lightning messages

[lightning-developer]

@t-bast noted in #765 (comment)_

This is a valid point, to be honest I really don't know whether we must try to keep update_add_htlc fixed length or not, maybe making it variable-length thanks to additional tlvs in the extension is actually a good thing as you suggest. That deserves more analysis.
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Adding random padding data in their tlv extension could make sense to ensure they're never fixed-size if we want to thwart this kind of analysis. That's an interesting area to explore.

I thought I would create a separate issue for this as that is not necessarily related to route blinding but to make traffic analysis via man in the middle attacks harder.

We would basically need to figure out:

• the upsides for making messages of variable length
• the effectiveness of protecting from man in the middle attacks
• a mechanism to do this properly
• the downsides of doing so