This implements an idea from Pieter Wuille <pieter@wuille.net>
https://github.com/bitcoin/bitcoin/issues/28722#issuecomment-2807026958 to
allow `bitcoin-cli` to connect to the node via IPC instead of TCP, if the
`ENABLE_IPC` cmake option is enabled and the node has been started with
`-ipcbind`.
The feature can be tested with:
build/bin/bitcoin-node -regtest -ipcbind=unix -debug=ipc
build/bin/bitcoin-cli -regtest -ipcconnect=unix -getinfo
The `-ipconnect` parameter can also be omitted, since this change also makes
`bitcoin-cli` prefer IPC over HTTP by default, and falling back to HTTP if an
IPC connection can't be established.
These are currently empty structs but they will be used to pass some
function and object pointers from bitcoin application code to IPC hooks
that run, for example, when a remote object is created or destroyed, or
a new process is created.
