In this commit we add a new type to the lnwire package: FundingFlag.
This type will serve as an enum to describe the possible flags that can
be used within the ChannelFlags field in the OpenChannel struct.
We also define the first assigned flag: FFAnnounceChannel, which
indicates if the initiator of the funding flow wishes to announce the
channel to the greater network.
This commit adds the new set of single funder messages from the
specification. As a result, after this commit and a follow up, all of
our messages will directly line up with those that are detailed within
the specification.
The new set of funding messages are very similar to our prior ones,
aside from the main difference of the addition of several channel level
constraints that give nodes control over their exposure, throughput,
and other values.
