List supported/required feature bits explicitly in ChannelManager

List supported/required feature bits explicitly in ChannelManager

Historically, LDK has considered the "set of known/supported
feature bits" to be an LDK-level thing. Increasingly this doesn't
make sense - different message handlers may provide or require
different feature sets.

In a previous PR, we began the process of transitioning with
feature bits sent to peers being sourced from the attached message
handler.

This commit makes further progress by moving the concept of which
feature bits are supported by our ChannelManager into
channelmanager.rs itself, via the new `provided_*_features`
methods, rather than in features.rs via the `known_channel_features`
and `known` methods.