lnwire: add new Sig type to handle conversion to/from btcec.Signature
lnwire: add new Sig type to handle conversion to/from btcec.Signature

In this commit, we add a new signature type. We’ll use this type to
avoid fully decoding a signature on the wire into a btcec.Signature.
This type is only really needed when we need to do signature
validation, as a result, always encoding it is a waste. Several helper
methods have been added to the new struct in order to ensure that we
can use it in the existing codebase without substantial issues.

aakselrod/lnd,LightningNetwork/lnd,aakselrod/lnd,LightningNetwork/lnd