test(gameserver): add gameserver test application

[andrewazores]

Welcome to Cryostat! 👋

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See https://github.com/cryostatio/cryostat/pull/402 for this image being deployed alongside Cryostat

Description of the change:

This change allows an environment variable to be configured so that...

Motivation for the change:

This change is helpful because users may want to...

How to manually test:

1. Run CRYOSTAT_IMAGE=quay.io... bash smoketest.bash...
2. ...

[andrewazores]

This should be buildable using build.bash, and testing that it starts up and the Agent connects using https://github.com/cryostatio/cryostat/pull/402 should be straightforward. Actually getting a game client to connect to the server instance is a bit different since the official game clients require an account signup and things, but there are third party launchers that allow connecting to custom servers without an account. Drop me a message if you'd like some direction on how to accomplish that, but this is only really needed if you want to try to generate some activity and additional JFR data. Even without connecting a game client, the target should appear in Cryostat and there should be data to gather.