This commit introduces the `CustomRecords` type in the `lnwire` package,
designed to hold arbitrary byte slices. Each entry in this map can
associate with TLV type values that are greater than or equal to 65536.
In this commit, we add a new ChannelType field as a new TLV record to
the OpenChannel message. During this change, we make a few tweaks to the
generic TLV encode/decode methods for the ExtraOpaqueData struct to have
it work on the level of tlv.RecordProducer instead of tlv.Record, as
this reduces line noise a bit.
We also partially undo existing logic that would attempt to "prepend"
any new TLV records to the end of the ExtraOpaqueData if one was already
present within the struct. This is based on the assumption that if we've
read a message from disk to order to re-send/transmit it, then the
ExtraOpaqueData is fully populated so we'll write that as is. Otherwise,
a message is being encoded for the first time, and we expect all fields
that are known TLV fields to be specified within the struct itself.
This change required the unit tests to be modified slightly, as we'll
always encode a fresh set of TLV records if none was already specified
within the struct.
In this commit, we create a new `ExtraOpaqueData` based on the field
with the same name that's present in all the announcement related
messages. In later commits, we'll embed this new type in each message,
so we'll have a generic way to add/parse TLV extensions from messages.
