Enable fewer messages than nonces

[matthewjablack]

Since DLC CET compression involves outputs requiring fewer nonces, this PR enables fewer messages than nonces to be passed into CreateCetAdaptorSignature and VerifyCetAdaptorSignature, which calculates the adaptor point based nonces up to the length of msgs array.

Numeric Outcome Details

[Tibo-lg]

Can you just rebase on develop and squash your commits? Also reviewing your code I noticed that there is a check lacking in ComputeAdaptorPoint, I think if you pass an empty vector of r_values it will crash, so it'd be nice to add a condition to the check at the beginning of the function. I can also add it later though, just if you're motivated :)

Also really nit-picking but if you could separate the change to the .gitignore in a separate commit it'd be great.