Adds an integration test for the file-based discovery plugin
to test the plugin operates correctly and uses the hosts
configured in `unicast_hosts.txt` with a real cluster
Closes#20459
TransportService is such a central part of the core server, replacing
it's implementation is risky and can cause serious issues. This change removes the ability to
plug in TransportService but allows registering a TransportInterceptor that enables
plugins to intercept requests on both the sender and the receiver ends. This is a commonly used
and overwritten functionality but encapsulates the custom code in a contained manner.
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
Simon Willnauer