This commit introduces a new plugin for file-based unicast hosts
discovery. This allows specifying the unicast hosts participating
in discovery through a `unicast_hosts.txt` file located in the
`config/discovery-file` directory. The plugin will use the hosts
specified in this file as the set of hosts to ping during discovery.
The format of the `unicast_hosts.txt` file is to have one host/port
entry per line. The hosts file is read and parsed every time
discovery makes ping requests, thus a new version of the file that
is published to the config directory will automatically be picked
up.
Closes#20323
Ali Beyad