Add Server-Sent Events

[mrtoirol]

This PR adds a /events endpoint to the API that users can use as an EventSource. This endpoint relays and broadcasts multiple Towny events in real time, using server-sent events (SSE) through Javalin. The complete list and data structure can be found in the docs.

[Warriorrrr]

This can be simplified, the result of the futures is ignored so combining them into one is redundant.

(requires adding a plugin field obviously)

Suggested change

	CompletableFuture.allOf(
	clients.stream()
	.map(client -> CompletableFuture.runAsync(() -> client.sendEvent(event, message)))
	.toArray(CompletableFuture[]::new)
	);
	for (final SseClient client : clients) {
	plugin.getServer().getAsyncScheduler().runNow(plugin, task -> client.sendEvent(event, message));
	}

[Warriorrrr]

I think that player join/quit events are too intrusive, this class should be removed

[wyattgill9]

I shadow Warriorrrr's opinion, those events are intrusive though, it can prove helpful to have it...

[Owen3H]

Imo, the event names could cause some confusion, for example "DeleteNation" sounds like the event deletes the nation.
Instead I think they could be a bit more consistent and convey what happened a bit better.

RenameTown -> TownRenamed
DeleteTown -> TownDeleted
NewTown -> TownCreated

RenameNation -> NationRenamed
DeleteNation -> NationDeleted
NewNation -> NationCreated
NationRemoveTown -> NationTownRemoved or NationRemovedTown
NationAddTown -> NationTownAdded or NationAddedTown

[Warriorrrr]

This PR has gone a bit stale but this is still something we're interested in having, feel free to re-open if this is still something you want to pursue (or someone else also can)