Expose Node status and statistics

[tnull]

We should provide a utility that returns some more information on the node status and statistics overall.

One example would be to which block height the node is currently synced.

[reez]

This would be great to not have to make an external API call if it gets exposed in ldk-node.

Block Height would be nice to have for small UI/UX things like:

• showing current block height
• showing at what block a channel was created on (ChannelDetails has confirmations, so could use block_height and confirmations to get to the block height channel was created on)
• showing other node status and statistics