Fetch our `InitFeatures` from `ChannelMessageHandler`

Fetch our `InitFeatures` from `ChannelMessageHandler`

Like we now do for `NodeFeatures`, this converts to asking our
registered `ChannelMessageHandler` for our `InitFeatures` instead
of hard-coding them to the global LDK known set.

This allows handlers to set different feature bits based on what
our configuration actually supports rather than what LDK supports
in aggregate.