Add easy-to-use support for running a node over CJDNS

[jonatack]

Alongside Tor and I2P, CJDNS is a complementary option supported by Bitcoin Core since v23 that can enhance network redundancy and robustness for both the Bitcoin peer-to-peer network and individual nodes.

Network redundancy is valuable. Both the Tor and I2P networks have seen sustained attacks, and the recent one against I2P pretty much took it down until the attack stopped -- and the attack could return. It is important to have backup networks.

I2P use by bitcoin nodes took off when this project added easy I2P support. It would be great to see the same for CJDNS to help it achieve lift-off.

More info:

• https://jonatack.github.io/articles/using-alternative-p2p-networks-with-bitcoin-core

• https://github.com/bitcoin/bitcoin/blob/master/doc/cjdns.md