This adds a new RPC method "getzmqnotifications", which returns information about all active ZMQ notification endpoints. This is useful for software that layers on top of bitcoind, so it can verify that ZeroMQ is enabled and also figure out where it should listen.
Backport of core PR13570 and PR13645:
https://github.com/bitcoin/bitcoin/pull/13570/files
https://github.com/bitcoin/bitcoin/pull/13645/files