Add minimum online checks for a server to be considered online

[nelsongraca]

This is a feature I need my backend servers start replying before initialization is complete and creates problems if a player joins too fast, this is a way to prevent that from happening, since with this along with the checkDelay allows me to set how much time to wait after the server is online.

Let me know if you want me to make any changes, one I would probably do is make QueueTestUtils.onlineServers return a Supplier and on the tests replace Set<String> onlineServers with var onlineServers

[AlexProgrammerDE]

LGTM!

[nelsongraca]

@AlexProgrammerDE thanks for the quick review, when are you planning to make a release?

[AlexProgrammerDE]

Right now this is becoming PistonQueue v4. I'd like to work on more things such as migration to MiniMessage, better placeholders, even cleaner code structure, etc.
I don't wanna rush a release because I need to do more testing and clearer documentation. I hope to publish an official v4 release in the first few weeks of 2026.