🔨 Add binding to force zones to stay away for arbitrary amount of time

[zach2good]

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Describe the feature

Downstream request.

zone:setKeepalive(utils.minutes(15)) or similar.

Currently we keep zones alive for 5 seconds or so after the last player exits before it goes to sleep. This binding would take whatever mechanism that is and set it to be an arbitrary timestamp that defaults to be "time the last player left" + 5 seconds, where the 5 seconds can be overridden temporarily. Once the zone goes to sleep, that time will be reset to the default and will need to be set again if you want a higher timer.

This is thought of as TEMPORARY!

In an ideal world the keep-alive-ness of zones will be managed exclusively from the world server, where it'll send "wake up" and "go to sleep" messages, but our RPC systems in both C++ and Lua land are hard to use and fragile.

Once they're enhanced or replaced with gRPC or something gRPC-like, with a layer of Lua bindings on top, then we can do away with these arbitrary timers.