brontide: add EphemeralGenerator as a functional arg to NewBrontideMachine
brontide: add EphemeralGenerator as a functional arg to NewBrontideMachine

This commit modifies the NewBrontideMachine constructor to allow a
caller to specify exactly _how_ new ephemeral private keys for the
crypto handshake are generated. This allows callers a bit more
flexibility when using brontide, and also allow test cases to insert
specific public keys for use within the hand shake.

AndrewSamokhvalov/lnd,AndrewSamokhvalov/lnd,aakselrod/lnd,ddustin/lnd,LightningNetwork/lnd,LightningNetwork/lnd,aakselrod/lnd,ddustin/lnd