Enhance AllocationSpec List modification to include delete operation

[joeholley]

Any plans to include the ability to delete a value from a list at allocation time? It's the only list manipulation missing from the allocationspec (but is in the SDK) and the Agones integration I'm working on would benefit from it.

Is your feature request related to a problem? Please describe.
Concretely: my PoC backfill matchmaking implementation sets a backfill request identifier Label at a pre-defined key on the game server so incoming matches containing backfill players can find a server which wants those kinds of players by using a LabelSelector. This works nicely since the AllocationSpec lets me remove the backfill request identifier using a metadata patch operation at allocation time, so as servers get their backfills they are atomically removed from the list of servers requesting backfills. This approach is fine as-is, but is limited to only one concurrent backfill request per server, since I have to use that pre-defined Label key in order to make the LabelSelector.

Describe the solution you'd like
If I could move to storing this ID in a List instead, then I could associate multiple backfill requests with the server (in order to for example, request backfills for tank players, healers, DPS concurrently), having the allocationSpec select on the presence of the ID in the List. Would be a nice-to-have feature, but to be feasible, this would require the ability to atomically remove the backfill ID from the list via the AllocationSpec, like I can with the Label value in my current implementation.

Describe alternatives you've considered
The current working PoC implementation is the forerunner for alternatives.

Additional context
This implementation is for example content slated to ship with the next version of Open Match.