Make 'Velocity required kick message' configurable

[Twarug]

This PR makes it possible to change a kick message send when connecting directly to a Velocity configured server.

[kashike]

If your server is configured correctly this message should never be seen by the public. For this reason, the message is not configurable.

[Twarug]

Not in every scenario it is possible to hide backend server, making it inaccessible to outside world. (eg. MC hosting providers)
By adding this configuration, owners in such situation, could redirect players to right IP address/domain.

[electronicboy]

exposing proxied servers to the public network is an unsupported configuration, and we are not inclined to support or provide configuration options which support or condone such setups.

if people so desire, there have been plugins over the years that would do that for bungee setups, I'd imagine those will also work for velocity if setup properly, but, at the very least, it is your own foot essentially.