Add helper functions to verify node and channel annoucements

Add helper functions to verify node and channel annoucements

Right now the only real way to verify the node and channel
announcements is to call `update_node_from_announcement`/
`update_channel_from_announcement`. If you want to do some
processing before you add to your network graph then you need to
manually verify the signature. This adds some nice helper functions
to make it easier.

I tried to do the same for channel update but it did not seem as
easy so figured that is fine to punt on since I don't see many
people doing manual things with channel updates.