Implement NKN blockchain provider

Hi,

I quote:
>NKN is a new kind of peer to peer network connectivity protocol and ecosystem powered by a novel public blockchain. It uses economic incentives to motivate Internet users to share network connections and utilize unused bandwidth.

More info on the [NKN](https://nkn.org/) site.

They are a bit of a unique "blockchain" because they aren't only a blockchain.
I wouldn't do it justice in trying to explain it so it's best to read their site.

In my experience, their blockchain is rather fast with transactions being done in seconds.
They allows for some custom data to be added in a transaction (i believe it was 200 bytes) which would allow anchoring i think. That kinda depends on how large the string is that needs to be anchored but i assume it's just one hash with probably less then 100 characters.

The transaction fee is nice too. Free :)
Well, won't be for years to come, but at the moment it's either free or super low.

Cheers,
Mark

Start with the linked NKN site to understand its transaction model, custom data limits, and fees. The issue names no files, tests, or entry points; done would require a defined NKN provider that supports the service's anchoring needs.

Written by the indexing model from the issue text.