Merge bitcoin/bitcoin#22079: zmq: Add support to listen on IPv6 addresses

e6998838e5 doc: Add IPv6 address to zmq example (nthumann)
8abe5703a9 test: Add IPv6 test to zmq (nthumann)
ded449b726 zmq: Enable IPv6 on listening socket (nthumann)

Pull request description:

  This PR adds support for listening on IPv6 addresses with bitcoinds ZMQ interface, just like the RPC server.
  Currently, it is not possible to specify an IPv6 address, as the `ZMQ_IPV6` [socket option](http://api.zeromq.org/master:zmq-setsockopt#toc27) is not set and therefore the ZMQ initialization fails, if one does so. The absence of this option has also been noted [here](https://github.com/bitcoin/bitcoin/issues/15198#issuecomment-617378512).
  With this PR one can e.g. set `-zmqpubhashblock=tcp://[::1]:28333` to listen on the IPv6 loopback address.

ACKs for top commit:
  laanwj:
    Code review ACK e6998838e5
  theStack:
    Tested ACK e6998838e5 🌱

Tree-SHA512: 43c3043d8d5c79794d475926259c1be975b694db4fcc1f7750a9a28e242f0fa1b531735a63ea5777498003aa5834f6243f39742d0f3941f2f37593d0c7890700

doc/zmq.md

@@ -84,6 +84,7 @@ For instance:
 
     $ bitcoind -zmqpubhashtx=tcp://127.0.0.1:28332 \
                -zmqpubhashtx=tcp://192.168.1.2:28332 \
+               -zmqpubhashblock="tcp://[::1]:28333" \
                -zmqpubrawtx=ipc:///tmp/bitcoind.tx.raw \
                -zmqpubhashtxhwm=10000
 
@@ -125,6 +126,9 @@ Setting the keepalive values appropriately for your operating environment may
 improve connectivity in situations where long-lived connections are silently
 dropped by network middle boxes.
 
+Also, the socket's ZMQ_IPV6 option is enabled to accept connections from IPv6
+hosts as well. If needed, this option has to be set on the client side too.
+
 ## Remarks
 
 From the perspective of bitcoind, the ZeroMQ socket is write-only; PUB