In this commit, the lnwire.NodeAnnouncement2 type is defined. This will
be used to represent the `node_announcement_2` message used in the
Gossip 2 (1.75) protocol.
To ensure that the RandNodeAlias helper can be used elsewhere to
generate random aliases, we adjust it in this test to only produce valid
UTF-8 strings as required by the spec.
In this commit, we add a new `TestMessage` interface for use in property
tests. With this, we'll be able to generate a random instance of a given
message, using the rapid byte stream. This can also eventually be useful
for fuzzing.
